Dreamcast: (No restrictions, any actor/actress can be used)
Batman - Christian Bale
Superman - Henry Cavill
Wonder Woman - Jaimie Alexander
Flash - Ryan Reynolds
Green Lantern - Bradley Cooper
Aquaman - Chris Hemsworth
Green Arrow - Charlie Hunnam
Cyborg - Idris Elba
Martian Manhunter - Rick Worthy
Current Cast: (with Additions)
Batman - Ben Affleck
Superman - Henry Cavill
Wonder Woman - Gal Gadot
Green Lantern - Ryan Reynolds
Flash - Bradley Cooper
Aquaman - Charlie Hunnam
Green Arrow - Jensen Ackles
Cyborg - Ray Fisher
Martian Manhunter - Rick Worthy
Alternates or Backup choices to current cast/additions:
Green Lantern - Bradley Cooper, Jensen Ackles, Chris Pine
The Flash - Ryan Gosling, Garrett Hedlund, Cam Gigandet
Aquaman - Nikolaj Coster-Waldau, Alexander Skarsgard, Armie Hammer
Green Arrow - Charlie Hunnam, Ben Foster, Norman Reedus
Martian Manhunter - Lance Reddick, Benedict Cumberatch, Dennis Haysbert
I'm not a fan of Martian Manhunter or Cyborg, so they would be supporting members of the League. I'd add Green Arrow as the first non founding member added to the League, I like that character far more than Cyborg or Manhunter (I hate MM). Hal Jordan and Barry Allen are my versions of Lantern and Flash regardless. Have no interest in the other versions. I'd prefer to see Ben Foster as a villain somehow, otherwise he might actually be my first choice for Green Arrow.
Since I doubt that Ryan Reynolds will be back as Green Lantern my top choice is Bradley Cooper, which would make Ryan Gosling The Flash. So basically Ben Affleck, Henry Cavill, Gal Gadot, Ray Fisher, Bradley Cooper (Hal Jordan), Ryan Gosling (Barry Allen), Charlie Hunnam (Arthur Curry), Jensen Ackles (Oliver Queen) and Rick Worthy (J'onn J'onzz) would be the most realistic group.