I'm all for a really diverse cast for the Batman universe. In the comics, basically all the main heroes and villains in Gotham are white. I think that it'd be cool to shake things up and update these characters a bit for the movies
Some ideas that I don't think would piss off the fans too much
Nightwing- Steven Yeun (he's pretty much exactly how I picture Grayson except he's Korean) Dev Patel would be another interesting choice
Mister Freeze- Giancarlo Esposito (Gus Fring in Gotham? Who wouldn't be down for this?)
Two Face- Denzel Washington (maybe he's too old, but I've loved this idea since it was first brought up pre-TDK. I've come to really like the idea of an African American Two-Face after Billy Dee was robbed of his opportunity)
Catwoman- Ruth Negga (there is precedent for a black Selina already. From Batman 66, to Halle Berry, to the Lego Batman movie. Ruth Negga would absolutely nail this role)
I don't think any of these ideas are diversity for diversity' sake, I genuinely think these are great actors that could nail the role just as much as if not more than any other actor
And hopefully next time we see Ra's and Bane, they aren't played by white guys. It didn't bother me in the Nolan movies because the performances were so good, but if they're gonna do it again, might as well try to be more accurate to the comics. I would go with Ghassan Massoud as Ra's and Edgar Ramirez as Bane.
Diverity in comic book movies is such an interesting topic. Racebending can be done poorly (Human Torch) or it can be done well (Deadshot). Some characters lend themselves more than others to racebending, and I (usually, it's case by case though) don't think fans are being racist when they complain about a race change. I think it's mostly that fans are just used to their favorite characters looking a certain way. But when it's done right, I think it's a really good thing. Most of us grew up loving superheroes and we looked up to them as kids. It's good to have superheroes and villains that look like us. I'm not saying a character has to look like you for you to relate to them, but when you're a kid, and a superhero kinda looks like you, it sends a message. It's a subtle reinforcement of the idea that you can grow up and be a hero too.