Good idea -- a tighter core cast, and a more diverse one. Still, I think I'd pick John Stewart over Cyborg. The JL should have a GL. And I think there'd be room for Flash too. The Avengers had six core hero characters, seven if you count Fury. (Although given that Hawkeye was relatively marginalized, maybe it's still six.)
I would have picked a Green Lantern if not for the recent flop and I originally wanted to include Flash as the sixth, but I decided that I liked five members better.
Avengers worked with more heroes but keep in mind that everyone was surpsised that Whedon managed to give everything something meaningful to do as well as he did.
The bigger the line up, the bigger the risk someone gets screwed and/or the movie suffers from trying to focus on everyone. I'd rather not take that risk, start with a smaller line up and when the first movie was a success, introduce one or two additional members in the second.