Not too much to tell right now, since I'm also trying to learn how to Ren'Py properly. But the gist of it is this: set in the Slave Trainer 3 universe, about a decade or so later than that yime period, your character (select gender and name) goes to "the city" to meet his/her parents, but they've vanished. You have to try and find them, explore the various places, each one with a different set of problems and characters, and avoid the pitfalls to become another slave. And possibly uncover the mystery at the heart of the city. Lots of factions: townsfolk, royals, nobles, priests, thieves, drug dealers, bandits, etc to act and react with. All of the usual reputation meters etc, gaining favour with one faction, means losing favour with another etc.
When I said it was in the flavour of Witch Trainer, I meant in terms of gameplay and art style, not necessarily using existing characters, although I am not averse to doing that either. I do like the idea of having characters from different universes all living in this one and maybe playing against type.
The big problem, beyond learning Ren'Py and trying to do everything myself (because I have no cash for it), is that I feel I am biting off way more than I can chew.
Anyway, that's what I have in mind, but it may never see the light of day. Aside from the above problems, stupid real life keeps getting in the way.