Batman, Superman, Wonder Woman and Harley Quinn are already confirmed, a pic showing part of the roster leaked.

Other safe bets:
Green Lantern
Flash
Green Arrow
Supergirl
Nightwing
Robin
Joker
Shazam
Aquaman
Lex Luthor

And possibles from other franchises:
Wizarding World: Harry Potter, Ron Weasley, Hermione Granger, Draco Malfoy, and Newt Scamander.
Lord of the Rings: Gandalf, Frodo, Aragorn, Bilbo, Gollum
WB classic cartoons: Bugs Bunny, Daffy Duck, Tazmanian Devil, Animaniacs, Scooby Doo (and all of Mystery Inc?), Tom and Jerry, Fred Flintstone.
Cartoon Network: Finn and Jake (Adventure Time), Johnny Bravo, Ben 10, Powerpuff Girls, Dexter('s Lab), Steven Universe.
Adult Swim: Rick and Morty

Probably also some HBO characters, most likely from Game of Thrones?