I'll put money on it not happening. Besides, even though JL has the more famous characters in the roster,The Avengers was there first, WB is only ever going to look like ripping off the concept, even though it's not that special a concept in the first place really.
Batman and Superman don't need origins, but I honestly don't think the GA knows THAT much about the others. Ironically, Bats and Supes will be the few who DO get an origin.
As for if Thor, Cap and The Avengers are successful, I think these things will happen:
1. More solo films for Cap, Thor, Iron Man, Hulk, Hawkeye, possibly that SHIELD movie
2. Avengers sequels
3. Marvel will try and get Spiderman, Xmen, Fantastic Four, Ghost Rider, Daredevil ex. back
4.Marvel will create solo films for lesser known characters
5. Marvel and Disney will create live-action shows.(Disney owns ABC I believe)
6. A Marvel Universe Theme Park will be built in Disney(much more in-depth than the current one at Universal)
7. A-list actors and A-list directors will be more interested in these films. Not going to make any guarantees they actually get involved
8. Disney Pixar will look into developing a Marvel animated film
9. DC will reboot Batman again to keep Nolan's films separate. These will be more farfetched and more comic booky than Nolan's films. Or they can avoid the reboot and just make a non-direct sequel where it is more comic booky.
10. DC will build on this new Batman franchise and create a DC Film Universe and eventually release a Justice League.