Another question is who do you have as the ultimate big bad that ties all this together? Do you go Darkseid and give Marvel the finger saying the original beats the copy (and hell a New Gods movie could be ****ing awesome).
Or do you just go super bat**** crazy and make it the Anti-Monitor and do a full blown crisis and cross over into the TV shows. (so make movie Flash Wally and he can meet TV show Barry).
I'd like Aquaman and his Atlantis army as the "villains" of the first Justice League film. Aquaman, of course, wouldn't be a real "villain", but just as Zod only wished to harm Earth/Superman to start a new Krypton, and just as Luthor wishes only to harm people/Superman to save Earth; I think that placing Aquaman in the difficult position of thinking/being manipulated into thinking that he had to attack the surface-dwellers to save Atlantis is a great way of continuing the "villains according to whom?" trend so far in the universe.
As indicated, Aquaman wouldn't be the real adversary; his brother, the Ocean Master would be the villain, with Aquaman joining the Justice League to defeat Ocean Master and his dedicated large group of rebel soldiers.
I think this works for several other reasons: 1. After Man of Steel (and the Avengers), I think it may still be too early to do another alien invasion. The second JL film should feature Braniac/Darkseid, however. 2. It's a good way to introduce Aquaman and his origins before his own film months later. 3. I really like Ocean Master, but the similarities between Aquaman-Ocean Master and Thor-Loki are obvious, so "doing away" with Ocean Master early is a plus, in my opinion. This way his solo films would feature Black Manta, the Trench, etc.
The Anti-Monitor would be a fantastic villain for the Green Lantern Corps film.