sounds like an exciting undertaking! i say start with your own joker, its a world you’re creating, so use yours but keep another live action in your thoughts as you progress in development there maybe a place for another one (possibly somehow doing a more live actionish version of Hamill’s).
I don’t what you’re thinking by way gameplay; but I’ve always loved Hush as a character, also Zsasz can be a real dark graphic storyline (Lector thoughts). Freezes tragic story, carrie kelly can bring some attention to the game and be awesome to finally have in a game. Court of Owls can bring in Dad. Knightfall’s got some great ideas for character arcs. Im kind of just throwing all types out there depending on the game you’re making (like a GTA or KOTR or something totally different). Just some thoughts. Hope they’re not dumb. Sounds awesome though!