Nicholas Hoult - Dick Grayson/Nightwing
Patrick Schwarzenegger - Wally West/Kid Flash
Being only 1 year younger than Ezra Miller, they could say that he's at least 5 years younger (Is Hollywood, they take 30 something actors to play 16 year old teens) and their relationship would be more like a older-younger brother relationship.
Nathan Kress - Garfield Logan/Beast Boy
Michelle Ang - Raven
Stephan James - Kaldur'ahm/Aqualad
Torrey DeVitto - Koriand'r/Starfire
Kevin Bacon - Brother Blood
Cameos:
Ben Aflleck - Batman
Ezra Miller - The Flash
Jason Momoa - Aquaman
Nick Robinson - Tim Drake/Robin