Man of Steel 2 needs to come before Man of Steel 3 :P Surely this movie can't be seen as Man of Steel 2 anymore. And that's not a bad thing!
Wonder Woman should be priority #1 after Batman vs. Superman. She'll obviously get to show her face in this movie, and hopefully (if the character is handled correctly), it will intrigue people to want to see more. She'll be at the height of interest... or at least as high of an interest as she's going to get at this point from the GA. A swift follow up with two years would be best if we want to see her franchise move forward.
Flash is the next priority. People have been crying out for a movie from him for some time now. Regardless of the size of his role in Batman vs. Superman, I think interest in him will come to a head in 2015 as well. If they could develop his movie in parallel to Wonder Woman, that would be spectacular. But if that isn't realistic, he should be next up in the queue. The GA has something of a short attention span when it comes to characters that aren't household names it seems. Chances are, no one knows who Steel or Spawn are since their movies have been so far removed without an update. Sure, Flash is better known then the two of them already, but the motivation to go watch a movie about the character will only last so long. Luckily, the TV show will probably pave some of the road for the character.
After that, anything along the lines of Batman or Superman are fair game. I think Man of Steel could use another solo installment before we see any more Batman. Since the cast of the DCCU is about to get very crowded for Mr. Cavill, he'll need to keep his head above water so that he isn't overshadowed by his co-stars. Another solo Superman could do just that - developed in parallel to Flash or right after.
Green Lantern is a mystery. Some people get excited about talking about a reboot and others say "keep him to Justice League only". I think both sides of that argument are a little drastic. If they stay on board with Hal (which I'm almost certain they will), take him in baby steps with a prominent appearance in Justice League. Then, if response is good, look into the reboot but on low priority. A Justice League movie where he is the center of the story might be an even better move if he proves successful in the first Justice League. He's got some pretty colossal villains that would be worthy of the Justice League's intervention. Parallax, one of the rogue Corps (Sinestro? Red? Black?) Or, if they really want to go down this path, seek out John Stewart. There are rumors floating that Denzel Washington is being pursued for the role of John in BvS. I call bull as of right now, but we'll see.
So after Wonder Woman, Flash, and MAYBE Man of Steel, smaller projects like Justice League Dark, Suicide Squad, Deathstroke, etc. could be pursued. If they tie into the Man of Steel/Justice League world, cool. But I'd be fine if they were standalone or at least what Guardians of the Galaxy will be to the Avengers movies - in the same universe, but not directly involved with one another. These movies would be perfect for the inevitable time when superhero movies lose some of their traction as far as audience interest goes. This team-up craze can't (and probably won't) last forever. But given time, it might pick up again... just while it's on the down low, the lesser known characters may find their time to shine.