Hey there! I know this is a small little group posting ideas, but I'm amidst other projects now that are waiting on others, and because ideas are part of what I do, I thought I'd brainstorm a bit while I was here! Of course in this brainstorm, I set out with large overarching goals that might need a lot of reworking or simplification to fit within the design goals, and might not be appropriate for the project at all. With that said, the only cost is time reading, so I hope it won't be too high a price. My wording here is written a bit train of thought, so statements such as "I think it should", etc, should not be taken as expectation, just how I express my potential ideas when I'm thinking to myself, and I didn't take much time to filter my thoughts too greatly only because as I started brainstorming, my brain decided it was a bit pent up so there's more material than I expected to write. Because this is a first post here, it is going to be heavy on concept and light on specifics. If any ideas are considered relevant, you can decide which ones fit the project and I can help flesh them out so long as I still have time to assist.
Thanks for a great post

I think items could work on multiple tiers. Ores, plans and enchantments should all dictate what guards and regimens can wield. Dusts, tools and coals are needed to create items. Dusts help enchanting, tools and coals are needed for creation and enhancement of weapons and armor. Each of them are used and broken as items are made, and are spoils of exploration, dungeon delving, expansion, bought on the black market or summoned with mana (assuming you have the connections or strength of magic, respectively).
Items themselves should grow with the players, allowing modification and diversity. For instance, a dagger found may only start with Sharp, granting a plus to damage, but it can be enchanted to do fire damage, or help the girl sneak through the dungeon or elsewhere more easily. Its 'sharp' may be upgraded to vorpal with a scroll, or changed to 'double bladed' with another. Base weapons and armor should be rare, and have limits to their upgrades, where as the upgrades for their stats should be more plentiful. Infusing weapons with new enchantments should be costly in both gold and mana, but offer tangible benefits in line with their costs . i.e. weapons should be useful even if 'better' options come along.
Alchemy/smithing we might add later, we've talked about that a bit already. It would require some changes in items system but it's a LOT less complex of a system than for example Arena/Exploration as far as the code goes.
As for items augmentations, I never really liked that concept in games generally and if it is going to be added, it'll be around 3.0 version I expect

Mana drain – This is a simple renewal mechanic made to combat the 'infinite gains' problem of the original Whore Master, and the core reason behind it. Eventually, girls will run out of mana they can meaningfully contribute. Though I do think there should be immortal girls that are immune to this – I also think they should be very rare and hard won. I also think they should be either very difficult, very pricey, or perhaps impossible to breed. This could be dependent on how far the player is in the game, the amount of resources they have, or the difficulty of game they've chosen. When girls run out of mana, they could still earn gold for the player – giving him the chance to convert gold to mana to keep themselves from going under. But the exchange should become less desirable the more gold is being converted to mana. The idea behind this is that eventually a player will do better to replace current girls with new ones (likely their offspring or newly captured girls).
Alternatively, the player should be able to add girls to a mana pool so that they can replenish more well trained girls. This would offer an effective drain for extra girls gained through exploration and conquest, without exploding population outward. Let's say they are brought to the dungeons and milked of their orgasms to filter that energy into the other girls. This is where the offspring mechanic ties into this system neatly. In this way, trained girls are at a premium, and if they are replaced it is more slowly with similar (or better depending on the effort put in) replacements, all without such a sharp rise in population with no drain to match. There are always girls in the lower levels feeding mana up to the 'useful' girls, and there will always be a need to get more girls of lower caliber, but endlessly cluttering up your brothels will girls would come at a price of lower gains (lesser girls pay out less and are wasteful of mana) thus the incentives work to keep populations reasonable.
Having mana and spells might also be a window for a tangible reason for alignment. Alignment makes dark powers 'orgasm denial torture', 'regenerating flesh' (cheaper version of the good aligned 'protective shield', in battle she endures the agony of being cut up but heals from it quickly with limits defined by the amount of mana in the spell, but at the same time must endure the psychological stress of torture in terms of morale in battle and mood/fear after the battle), etc. Out of battle, having protective shield can protect your girls from violence, whereas regenerating flesh can let sadistic clients torment girls for their own amusement, or let them endure through more prolonged torment designed to break them in the dungeons.
We already have 4 delimiters in the game: AP, HP, MP and Fatigue. Adding yet another one is out of question at the moment, we have to many stats to work with as it is. I don't see draining AP/Fatigue from lesser girls in favor of better once either, maybe something like throwing a girl into the coma with small chance of death in order to create an item that can double the amount of AP/Fatigue for a day for any character.
There should be an option for girls to offer themselves willingly into slavehood. Doing so should require a complex, long chain of actions, and be rewarded with benefits such as improved stat growth, greater mana, etc.
After Alpha release, one of the first things we'll work on is a decent slave training system that will prolly require new stats, improved classes system and so on. I don't know about girls willingly go into slavery, doesn't seem important enough to write complex interactions/path, especially right now when there is so little content in the game.
Time skips. The player should be able to send children into an alternate dimension to be raised. Thus breeding superior women based on their parents stats without the game needing to take untold weeks for the girls to be viable. Also, you could direct the girls training every few week (which for them would be a much longer period of time) to help improve their stats in certain ways - so long as you're willing to invest the gold and/or mana it takes to get the effect you desire.
This is kinda brilliant

I've been thinking of a decent way to accelerate child growth for a while, this seems like a very good solution. We'll use it after pregnancy is added.
Strategy – Expansion should be a long, slow slog that comes in several phases. First, building up gold and mana should be necessary for a long while. War cannot be fought without basic resources. Building up critical mass of resources should be necessary for every phase of the war, which will be a large mana and gold burn. At the end of it however, you have expanded your front line outward, which means more area to defend and more resources to expend to do so, but at the same time, a larger area to cultivate and capitalize upon.
Conceptually, this means building up regimens (the analogue to gangs in whore master), training them and growing their ranks, equipping them with items infused with ores and enchantments found during exploration and dungeon delving. It means, if possible, having girls out there on the front lines who are experienced and able in combat. It means ideally having your player commander equipped with rare spells and the mana needed to cast them on the field of battle.
Moving forward on the field of battle will be a slow move, week to week. Injuries will be mitigated by healers within the gangs and more effectively among your girls, all costing mana. Any casualties that are injuries may return next week or in the coming weeks, whereas deaths cannot be replaced. Once you commit yourself to a battle, you must find a secure defensible front (fort or castle). Failure to do so means you are driven back to your territory, losses incurred without any appreciable gains. Success on the other hand requires a significant resource drain... that goes along with new opportunities.
Capturing new territory should not be the end of the struggle. Each town, fort, wooden pallisade, forest hold or castle should have a holed up elite guard that has the mana to conjure the food and resources they need for what is essentially indefinitely. The player must then infiltrate with high risk to a few elite girls, fight in the streets and then floor to floor in a direct conflict, or slowly win over the people, surviving the countless rebellions and magical strikes from within the stronghold until the people accept you.
Either way, whether they come out to negotiate, get kidnapped out, or imprisoned after a mighty battle, your job is then to legitimize yourself within the area by 'marrying' yourself (either enslaving or entering into a one sided polygamous relationship with) the civic leader, the duchess, the princess, the queen, or all of them. Each is harder than the next, and each gives a bonus to mana for breaking them or seducing them and making them part of your kingdom.
Training all of them under your banner will, instead of the rest simply falling in line, give your player the chance to unlock secret family heirlooms, artifacts and ores that can teach your player spells, enhance your regimens, or empower your girls, as well as having royalty that may make excellent whores for the wealthy elites or, if you want to milk their mana faster at the expense of efficiency, given to the masses to rut like animals for much less money but much more immediate mana.
I don't see this happening in PyTFall, maybe in Alkion... It will be to awkward. If someone is still working on this game in 5 years, maybe it can be some form of a "second stage", after the main part is completed, but it's not productive to go there right now.
Dungeon delving should be an equally in depth experience. With the chance to have girls captured as real as the chance to capture girls. There should be ways to quell the influx of monsters into an area so it can be secured, sealed from the more dangerous parts of the dungeon, and used to contain girls for torture or for training. There should be boss monsters that take either weakening through consistent battles, undermining through cutting off the flow of monsters, or goading into a challenge by stealing his women out from under him. Once you defeat all bosses of a level, you can go deeper.
However, each new level is not a stairway but a portal, and choosing to put a small amount of mana into the portal will open you to another small level with very little usable room for girls and very few usable dungeons tools. A larger mana contribution yields a much larger dungeon with more places to hold women and more effective training tools, but also a much longer, harder fight to clear that level. Choosing to hold a level requires ongoing mana expenditure to keep it attuned to the manor. Otherwise, the player can choose to let it slip when they move, draining it of resources and moving on to a new random dungeon.
That's about how we're planning to go about it. There will be no girl loss/capture before the slave training module, maybe there will be based on capturing girls for other trainers/market but not before Alpha. Torturing chambers for captured explorers/party members in the dungeons we'll use

Rest we've talked about already I think.
Girls, customers, and ongoing happiness.
In whore master, an outside observation (this could be completely wrong) is that satisfaction seems to be based on only a few things, whatever the customer is doing with one girl, her talent and attractiveness, and the cleanliness of the place. It might add depth if there were more factors. Perhaps you would get more out of a customer if they had a talented waitress serving him first, they ended up gambling away some of their money, and got a show before eventually entering the brothel.
This would give incentive to train many girls for many occupations that don't earn you much at all by themselves, but become necessary for higher paying clients and to be seen as a respectable establishment at higher levels of success. This could be another use for random girls, rewarding that excitement of random chance in seeking out and finding new girls to add without drowning the game in resource machines.
That already works to a decent degree and will farther be improved in the future. Jobs logic can be layered and improved indefinitely but if truth be told, I am beginning to doubt that it's a correct path, people seems content with really basic jobs in a lot of other games and prefer playability, diversity and content to logical depth.
New girl occupation: Talent scout – Having a loyal enough girl with high intelligence will unlock this option, letting you assign a girl to go through the world and seek out other girls based on your expectations. However, higher expectations lead to longer search times (depending on her stats, experience with the job and morale), and will often come with complications. A girl with high magic may be in a wizard school and have powerful friends to keep from you or to try to take her back if you seduce her away, she might be a priestess who is chaste and impossible to seduce, who has no interest in leaving so you have to put her temple in crisis so offering herself up is the only way to get her, etc.
I think Dark's proposal of Recruitment agency made more sense, but it's an option.
I think in terms of expenses, the player should be very limited. Perhaps his manor was destroyed and nearly driven away from all other planes of existence in an attempt to banish him. It took most all of his power to keep from being separated from the other planes, but that means he can only keep the manor open for half the day. A> This gives an enforced "Rest" period to help new players and challenge more experienced ones who power through B> it gives a use for the 'mana' mechanic. Spending mana allows you to do one of many things. Maintain a stronger link to the material plane. (allowing girls to work day and afternoon, then later day, afternoon, and dusk, then finally day, afternoon, dusk and the coveted nighttime if we wanted to expand the day/night cycle even further, but that's something that's a bit more tricky as working 4 times a day must be balanced with the extra maintenance and income involved).
I didn't get this but sounds overcomplicated and restrictive, something we've agreed to avoid.
As an aside for shops, it might be useful to have the ability to place an order for certain items. You pay a premium, but you don't have to wait on the store to stock it.
This might become easier once items system is finetuned.
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I do welcome ideas and proposals at all times, real help however would be commenting on game elements that are already in place or writing concepts/cementing in concept of game elements that are at hand or are planned to be implemented soon. Otherwise, thanks again for a great post, couple of ideas we'll definitely use!