Suggestions by @legendhero

Steve Blum

as Batman (Post credit scene) in The Flash and The Green Lantern

Jeffrey Combs

as Sinestro in The Flash and The Green Lantern

C. Thomas Howell

as Eobard Thwane/Reverse Flash in The Flash and The Green Lantern

Dee Bradley Baker

as King Shark in The Flash and The Green Lantern

Roger Craig Smith

as Chris Smith/Peacemaker in The Flash and The Green Lantern

Will Friedle

as George Harkness/Captain Boomerang in The Flash and The Green Lantern

Darren Criss

as Floyd Lawton/Deadshot in The Flash and The Green Lantern

Eric Bauza

as Harrison Wells in The Flash and The Green Lantern

Paul Dobson

as Joe West in The Flash and The Green Lantern

Cree Summer

as Amanda Waller in The Flash and The Green Lantern

Jason Griffith

as Guy Gardner/Green Lantern in The Flash and The Green Lantern

Ashley Johnson

as Carol Ferris in The Flash and The Green Lantern

Lizzie Waterworth

as Katlyn Snow in The Flash and The Green Lantern

Sam Vincent

as Cisco Ramerez in The Flash and The Green Lantern

Steven Yeun

as Roscoe Dillon/The Top in The Flash and The Green Lantern

Jason Griffith

as Leonard Snart/Captain Cold in The Flash and The Green Lantern

Kristen Li

as Iris West in The Flash and The Green Lantern

Josh Keaton

as Hal Jordon/Green Lantern in The Flash and The Green Lantern

Scott Porter

as Barry Allen/The Flash in The Flash and The Green Lantern

Brian Drummond

as Steppenwolf in Wonder Woman: Warrior of Peace