They couldn't do that because the Avengers's movies, X-Men/Wolverine movies, and Spider-Man movies are all under different contracts to different studios. Once Sony stops making Spider-man sequels (they are going to reboot, so it looks like never) and Fox stops making X-Men (they are making First Class, so it looks like never too), then Marvel gets the rights back to the character for movies. Once Marvel gets the rights back for all of their characters and can choose a single movie studio for all of those characters, then there's a possibility.
 
Back when Marvel signed the contracts to Fox Studios for X-Men (and spin-offs like Deadpool movie) and Sony for Spider-Man (and spin-offs), the company wasn't doing very good and the success of comic-book movies wasn't all that great. Now, they have to wait until the movie studios stop making movies with those IPs until they can take back their characters and give them to another studio.

I think a New Avengers spinoff film makes more sense then overloading the Avengers film with too many characters.

I'd like to see the New Avengers movie team including Black Panther, Vision, Ms. Marvel, Namor, and Deadpool.

On top of the aforementioned copyright issues, the story that's been set since after the battle for NY looks like it's already in motion. If there's going to be a "New Avengers" team, a) they won't be making an appearance until well after 2015, and b) it'd consist of entirely different members from the comic book team. I think Nightwing28710's dream cast is something that'd be likely to happen if the guys at Marvel/Disney were to approach the New Avengers right now.