The way I see it is like this:
Imagine that all universes have four dimensions: length, breadth, depth and time. Imagine you can collapse that universe down into two dimensions, so that all the realities there are float around in 3D space, like loose pages blown on the wind. OK?
Now the trouble with Mundiga is that bits of the page that universe resides upon have been torn. There are loose strands that fray and blow in the wind outside the plane of the main universe. Sometimes that means they brush up against other pages, other universes. That's when a portal opens and people and things can fall out of their own reality and into Mundiga.
The whole of Crossgate is slightly frayed in this sense, particularly the marketplace for some reason. That's why pretty strangers are so often found wandering there, dazed and confused. The catacombs on the other hand are very bedly frayed. They're flapping around loose in higher space which is why they seem to constantly shift and change, and they intersect many, many worlds. In fact they'd probably have detatched from Mundiga entirely save that some long dead archmage worked out a spell to bind a portion of catacomb-space to normal space-time. Or what passes for normal in Crossgate, anyway.
So you get a portal at the start of the game: it leads into the catacombs where all manner of weirdness may be found. There are also transient portals that open in the city at large. And it's by no means impossible that at some future point, modders will be able to create items or rooms that open custom portals to specific worlds, either to summon girls, or as the setup for event sequences.
You could get a good setup for a city mod from that...
*******
One day, the PC leads a catacomb mission. No big deal. Got to do it from time to time; keeps goon morale high and shows that you're not asking them to do anything you're afraid to do yourself. This time however, things go wrong, and instead of a portal opening and pulling someone into the catacombs, one opens the other way and drops the PC into a different world.
You find yourself in a high-tech world. Science rules, magic is weak to non-existent. If the player wants to get back home, he'll need someone to build a machine that can generate a space-time bridge back to Crossgate. To raise sufficent resources for that, he's going to need to dominate the city, like he did in Crossgate.
But it's not going to be easy. After all, the rules are completely different here. Still, there's one universal rule he understands well: there's always a market for sex.