In my head rent-free lives a Gotham/Birds of Prey fusion that rewrites the film to include the (now much older) Gotham characters.
Just imagine well-established King of Gotham and owner of the Iceberg Lounge Oswald Cobblepot outraged because, “Who the hell does this young upstart think he is?!” while Ed looks equally affronted in the background.
Then Barbara “retired queenpin” Kean strolls into the Iceberg Lounge like, “I knew his dad. He was a dick.”
Nygmob end up aiding Harley and the BoP in their mission against Roman for their own personal interests.
Along the way, Barbara Lee Gordon gets involved with the burgeoning Birds of Prey as Oracle as well, which leads to Barbara running into old flame Renee Montoya just as she’s giving up police work for a life of vigilantism (*bangs fist on table* middle-aged sapphic second chance romance!!)
Maybe David Li from Under the Red Hood replaces Zsasz as Roman’s right hand man, or maybe it’s Gotham!Zsasz who’s come to work for him and shifted loyalty (because Gotham!Zsasz/BoP!Roman would be fire, I will entertain no arguments).
Anyway, they eventually take down Sionis (maybe not take out, because Roman is too much fun) and then all go have celebratory drinks on Oswald at the Iceberg Lounge.