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There are two cities: the above city, and the below city. The above city is clean, and bright, and reproduction is clean and entirely divorced from physical intimacy: it's done by cloning, and genetic refinement. These people are aware, of course, that scrubbing certain traits from the gene pool might remove evolutionarily viable genetic features.

Hence the below city.

There, they toss people with wild genetic codes, naturally reproduced children, and newcomers to their worlds. There is a virus down there, one that causes constant tingling pain if left unabated. How does one abate the effects? - By having sex, of course. Stimulation to sexual pleasure centers of the brain will springboard your immune system, until you catch a new version you don't have immunity towards. In this way, the above city constantly stimulates sexual reproduction in the below city, ensuring that traits for survival are perpetuated.

The below city is in chasms, and in caves, underground.

As far as gameplay goes: people would be able to app characters the ordinary way (some experiment in the above city has brought in new genetic material for gene pool diversity, they're tossed into the below city to get that going) or they would be able to app native characters (form the above city or the below city). PCs would be subjects of a particular experiment above, tracking and following a certain group of people as they move through the larger city. In that way, they'd be brought together by the experimental staff.

There would be a lot of dark stuff going on. Experiments and alterations on characters (opt-in, of course), tinkering of sexual preferences in terms of kinks and wants, a lot of crime and violence in the below city. People could work to cure the disease, escape the below city, take over the below city from the crimelords that rule there, overthrow the above city - really, whatever they want to do.