Well, we have Captain America and Iron Man in production. The Avengers wouldn't be too outrageous. They just released an animated movie of 'em.
The problem is that now Marvel provides 100% of the funding to their movies. Before, they wouldn't pay a dime to fund the movies and made a few million if the movie was a success, while only losing a couple hundred thousand at the very worst if the movie totally bombed. Now, they can make many many millions off of a blockbuster hit, but will also be vulnerable to a wallet harpooning should any new movies totally fail.
It's not as safe as it used to be, rolling titles into theaters on a whim and etc. If Marvel decides to make a film, they need to absolutely make sure that it will be a box-office hit.
As for DC, I really don't know. I doubt they would ever make a JLA movie. Still, they would have to introduce all of the characters in their own movies beforehand. If even one of those movies tanked, I doubt they would return for the League, and, if they are all hits, it would make more sense to produce a sequel then to create a mashup.