I messed around with a lot of old Maxis games as a kid and kind of expected more would come out that would teach me how the world "really worked" in a way I could use to decide how to live my life. I still don't know what that would be like, exactly, but I figure *somebody* does, so I'm making a tool to help them, whoever they are: the Life Simulator Engine, LiSE, pronounced "lies".
The core feature is time travel. The development kit for Inform 7 had this, and it seemed so obviously useful that I needed to have it. This replaces save files; now, everything that ever happened in any playthrough of your game lives in a database.
There's also a rules engine, similar to the one in Ren'Py's Dating Sim Engine. By compartmentalizing the complexity of your game's rules, it should make them easier to reason about, and there's a way to time travel to just before or after any particular rule, to see if it did the right thing.