Firstly, DC will need to pick one of two directions. They can either make serious films or make silly ones. (It would be too difficult to copy the balance of seriousness, action, and humor that Marvel has without appearing to outright copy Marvel.) Based on the success of Nolan's Batman trilogy, it seems like that could be a good route to take, so I'll pursue that with this train of thought.
Who exactly is in the Justice League will depend on a few things. You'll want heroes who are recognizable, and cool, but also who aren't too far out of scale with one another. Superman is obviously a must-have in a DC property, so that sets the bar pretty high as far as power level goes. Green Lantern has the right power level for that, but it would have to be Hal Jordan, and that would likely require a reboot if we're to take him sufficiently seriously.
Here's a short list for consideration:
- Recent iterations of Aquaman are actually cool and he's pretty badass; in addition to his own (admittedly limited on this scale) powers, he's the king of Atlantis, and thus commands an entire submarine navy whose technology appears to at least match that of the surface-dwellers. The main risk with using him is that older people will remember him from his presence on Super Friends.
- J'onn J'onzz, the Martian Manhunter, is actually a prime target for a film adaptation these days. Between CGI and motion capture, a greenish-skinned shapeshifter could be done without breaking suspension of disbelief.
- Batman is a problem. He's definitely cool, but it's hard to scale him up without disturbing the coveted suspension of disbelief. It might be necessary to leave him in Gotham for this.
- Captain Marvel is a problem, but for different reasons. Aside from the whole name thing, he's basically a magical counterpart to Superman as far as powers are concerned. Being a match for Superman is one thing, but actually matching him for most of his powers (strength, speed, toughness) is another matter.
- Captain Atom could use some screen time. He'll be especially stand-out if DC doesn't give Superman heat-vision. Go nuts on the bloom.
- Wonder Woman could fit if DC could figure out how to dress her. But, hey, it's not like we actually know what the ancient Greeks wore into battle or anything. (Pro tip: Say the phrase "ancient Greek lasso" out loud.)
- The Flash could work if DC ensures that he's faster than Superman.
- If audiences are OK with someone who's expressly a magic-user, then DC could do a lot worse than Dr. Fate.
If DC goes the sillier route, then here's a short list for that:
- Hal Jordan Green Lantern, as-is
- Wally West Flash
- Aquaman (silver age style)
- ... Wonder Twins? I don't know, just grab a handful of supers and be done with it.