First things first… Line up for the Justice League. DC needs to compete with the Avengers. This thing grossed 500 million dollars. We need mainstream characters everyone likes and is familiar with, this is not the time to be “creative” and bring lesser characters to the screen, that’s reserved specifically for phases 2 and 3.
Batman, Superman, and Wonder Woman are a given. I’m not going to even try and recast these characters because re-casting a main character multiple times sets yourself up for failure (note: This is why the Hulk and Punisher aren’t as good as other franchises.)
Batman/Bruce Wayne- Ben Affleck
Superman/Clark Kent- Henry Cavill
Wonder Woman/ Princess Diana- Gal Gadot
The next part is much trickier. You need at least five members but no more than seven and the characters I would pick from are: Green Lantern, Flash, Cyborg, Aquaman, Martian Manhunter, and Green Arrow.
Now Green Lantern is a smart choice if you want space tie in plots later, but it doesn’t mean that you have to stick with Ryan Reynolds. My ideal Lantern (depending on the Lantern you choose to portray) would be the following:
Idris Elba as John Stewart
Alan Tudyk as Guy Gardner
Then the matter of deciding on the Flash, he’s an essential part of the team as both comic relief and the audience’s guide to everything. Also there’s the matter of which Flash, but chances are they’ll go with Barry Allen.
James Roday as Barry Allen
Or Ryan Gosling
Now this is difficult but despite the fact that Martian Manhunter is a great character, he’s not mainstream enough for most audiences to know who he is. Casting an Aquaman could be interesting to match Marvel’s royalty of Thor and you could even go a similar route in casting a relatively unknown actor. However, Josh Holloway might be an interesting choice.
Cyborg and Green Arrow are both kinda pointless at this point but between the two I would cast a Cyborg over GA because of overlap with the show Arrow. It might confuse audiences to have a different actor playing the movie version of the same character.