well I’d start with what time Batman is at like;
Early days
Good few years, experienced
Hardened older and wise
id say just write out as many ideas as you can for each one of the above and see what works well. Like in his early days the power is down all across gotham and at the source of it, for example hush is the one controlling gotham and working in the shadows pulling all the strings and the game could be on how Batman deals with this and how he adapts to his surroundings.
When he’s well in to being the Batman it could be where he believes he knows gotham and he has all crime under control and in comes the joker
Even if you want check out some Batman comic arcs and if you see one you like and you feel thats what you want to make your game on go ahead but make sure you put your own twist on it and really make it your own 😄
AS FOR JOKER
Make him look scary. Then put your own twist on him menacing, crime boss, insane etc