Depends on what you're after; there's quite a few fun Twine/SugarCube-based games around (the same engine as Free Cities, though most are much less technical), but not many are on the same scale.
For pure scope or timesink potential, Degrees of Lewdity by Vrelnir might work for you.
Other than that, off the top of my head, the Masculine Mystique by Barabbas over on TFGames tends to get pretty universal acclaim (and deservedly so). It's unfortunately unlikely to ever be finished, but what's there is fantastic. There's also Perverted Education (lots of sissification) and The Company (hypnosis and mind control). Both of those are available on TFGames as well.
A personal favorite is Imperial by Sherlocho; lots of really nice writing and worldbuilding, but unfortunately also dead in the water.
I'd also like to mention Academy of Fetishes and Fool's Quest, though be aware there's some pretty extreme content in those. Nothing underage, thankfully, but pretty much every other type of hard-no for a lot of people.