Suggestions by @remkosel

Emma Watson

as Iris West in The Flash

Joe Cole

as Lenord Snart in The Flash

Lucas Till

as Barry Allen in The Flash

Alexander Ludwig

as Aquaman in James Gunn's DCU (no mainstream fancast)

Hunter Doohan

as Flash in James Gunn's DCU (no mainstream fancast)

Nadia Hilker

as Wonder Woman in James Gunn's DCU (no mainstream fancast)

Andrey Ivchenko

as Rhino in Sinister Six 2027

Ben Hardy

as Venom in Sinister Six 2027

Milly Alcock

as Gwen Stacy in Sinister Six 2027

Steven Yeun

as Mister Negative in Sinister Six 2027

Matthias Schweighöfer

as Mr.Immortal in Great Lakes Avengers

Bill Hader

as Flatman in Great Lakes Avengers

Hannah Walters

as Big Bertha in Great Lakes Avengers

Anna Kendrick

as Squirrel Girl in Great Lakes Avengers

Cillian Murphy

as Doctor Doom in Earth-616 Illuminati

John David Washington

as Black Panther in Earth-616 Illuminati

Alexander Skarsgård

as Variant Thor in Earth-616 Illuminati

Tom Cruise

as Superior Iron Man in Earth-616 Illuminati

Sadie Sink

as Scream in Actors who should be in / join certain Cinematic Universe

Björgvin Arnarson

as Black Kid Flash in DCU Flash Trilogy