Joker 2 as a multiverse movie instead
Cut the musical stuff out for a multiverse plot (Joker awakens in a mishmash of the DCEU, Nolanverse and all DC movies and has to enlist the help of a Batman to get him back to the 80s). like a closure/celebration of all DC movies (before Gunns DCU movies start)
Margot Robbie as HQ instead of GaGa
Christian Bale as Batman in supporting role
Add various surprise cameos from previous DC movies, almost happened DC movies etc (Jim Carrey Riddler, Arnold Mr Freeze, Halle Berry as Catwoman, Ryan Reynolds as Green Lantern, Keanu Reeves as Constantine, Josh Hartnett as Superman, plus RDJnr as a Batman variant)