I am sure that I've missed many answers. And there are others I found but intentionally omitted because they were just very obscure. But feel free to throw out any that you think of that don't appear and I might consider updating.
Ron Perlman's in that one, too. He voiced Deathstroke.
Jesus, you were right. This is neverending.
Jennifer Connelly plays Chiren in that, and she also played Betsy Ross in Ang Lee's Hulk.
Ed Skrein plays Zapan, and he previously portrayed Ajax in Deadpool.
Jackie Earl Haley is listed as playing an undetermined role in Alita, and he also was Rorschach in Watchmen.
Christopher Waltz is set to play Dr. Dyson Ido, and he previously portrayed Benjamin Chudnofsky / Bloodnofsky in The Green Hornet - though the Green Hornet was originally a radio program and only later adapted in to a comic book so I may discount that one.
I also deleted the role of Green Lantern for Common since I can't find any confirmation that he's actually been cast... and the DCEU is falling apart who knows if they'll even make that movie.
David Harbour played Hellboy and is set to be in the Black Widow movie.
Rachel Weisz was in Constantine and set to be in Black Widow
Angelina Jolie is going to be in the Eternals and was also in Wanted.
Mahershala Ali was in Alita as Vector, did the voice for Uncle Aaron in Into the Spider-Verse, was Cornell "Cottonmouth" Stokes on the Luke Cage TV series, and was just cast as the new Blade.
Dolph Lundgren (also played The Punisher)
Patrick Wilson (also played Nite Owl)
Colin Farrell was cast as the Penguin. He was also in Daredevil.
Andy Serkis will play Alfred. He's in the MCU.
Zoe Kravitz will play Catwoman, who she also voiced in Lego Batman, so there's another hint for her.
If graphic novels are included, then Paul Dano can join because he was in Cowboys and Aliens and he was cast as the Riddler.
Another name I found was Peter Sarsgaard, pretty obscure but he was in Green Lantern as Hector Hammond, so feel free to decide whether he's in or not.
Also, I recently learned that Christian Bale is in talks to join Thor 4, so I'll be on the lookout for that one.
Overall, awesome quiz!