I've been going through the code, fixing warnings and mildly updating formatting style -- it's full of old-style function definitions with the types listed separately -- and I'm wondering what kind of features could feasibly be added.
So far I've got that palette inversion thing, (RemapColors rcZAWARUDO [range]), which needs more testing, and an optional fourth arg for DisplayPic to control transition speeds, confirmed to work on dsOPEN_CHECKBOARD and dsOPEN_FADEPALETTE (names from memory because lolphone).
Phil has suggested:
* Savegame thumbnails, though I've no idea where to even start, plus there's the whole palette thing.
* Masked sprites.
* Coroutines, so very likely.