DarkTI is right, and you can see it in other games already (every one adds something new, like hospitals and cinema studios). It gets boring with only brothel management, cause you doing all the same stuff over and over again, only with different girl packs.
I though of game world where you have live economics (simulation of it) with different resources and market, with countries and cities. It can be not like Skyrim, but like Pharaon and Cleopatra or Zeus, where you only see them on map, and cities are only buttons with text info (not very hard to implement visually). And you can trade with them, fight etc.
Xela's one of main features, on my opinion, is arena. Its good, cause, as I said before, its boring, if its only girl/brothel management.
NB: to think of tags. I had idea, will it be interesting to set tags depending on which directory it is? Example: we have root directory that contains all of images, and they placed in folders that named as tags, like root/girl_name/waitress/offended_by_visitor or root/girl_name/sex/bj. It can make setting tags to groups of images and adding new images much faster. Of course, it just an idea.