Obviously, I'd have Cavill and Affleck return as Superman and Batman, respectively. Unless Affleck is bad as Batman, which would shock me.
Wonder Woman is a bit difficult to cast. Jaimie Alexander would be perfect for the role in my opinion, but she is already playing a similar character in the Marvel Cinematic Universe. I'd be surprised if she jumped ship while still under contract with Marvel, even if Sif does have a minor role in the franchise. My three other picks for Diana are (in no particular order) Gemma Arterton, Adrianne Palicki, and Olivia Wilde. Palicki was the only great thing in that poor WW pilot, and she should get another shot at the role. Arterton is an overused name, but her exotic looks and action movie resume make her an ideal candidate. Wilde needs a big franchise role (beyond Tron), and she's interested in Wonder Woman.
As for Green Lantern, I'd introduce John Stewart first before bringing back Hal Jordan. My two picks for the former are David Oyelowo and (you guessed it) Idris Elba. Oyelowo list an up and coming actor who I'd love to see in a superhero movie. Elba, while also under contract with Marvel, still wants to play an actual superhero. Plus, he recently expressed interest in the John Stewart role. If they go with Hal again, I would consider Zachary Levi. Like Elba, he is signed to Marvel, but both have such minor roles that they could easily branch out into bigger characters in the future. If not Levi, I've recently thought about Emile Hirsch for Hal, though I'm not completely sold on the idea. Too bad Nathan Fillion's too old now.
If Grant Gustin impresses as Barry Allen on Arrow and they do decide to merge it with the DCCU, I'd be happy with him playing Flash in an eventual Justice League film. If the two are in fact separate, I have two other options: Chris Pine and Scott Porter. They look just like Barry in the comics, and are both in their early 30s. However, Pine may be too busy by the time Flash/JL come around (possibly due or another Star Trek sequel and/or Jack Ryan sequels, if the latter is a success). Porter, meanwhile, is a big comic book fan and is interested in this role.
I would cast Benedict Cumberbatch as Martian Manhunter. He is an actor who needs a superhero role at some point, and MM is an underrated hero that Cumberbatch would fit well. I'm not big on the idea of Cyborg in a JL movie, but if he is, Michael B. Jordan is the man for the job (even though he's probably playing Human Torch). Finally, I would consider Alexander Skarsgard, Armie Hammer, and Joel Kinnaman for Aquaman, who is another difficult character to cast properly.