Does Marvel even own the movie rights for the Fantastic Four? I know they don't for Spider-Man (Sony Pictures does, hence the reboot), but I can't find any decisive evidence. The two films were done by 20th Century Fox.
Don't know.
I do know the Avengers and the associated origin movies have all stayed pretty damn true to the comics. A few changes to allow the stories to be "modernized" but no changes that completely goes against the comics. In the comics the original Ironman had the same problem. Shrapnel trying to force his way into his heart. He had a "vest", and not a little LED light on his chest hooked to magnet but the concept is the same.
Fantastic Four and Rise of the Silver Surfer were both decent movies. Not great, but close enough to the original comic story lines. Could have been done better, but they sufficed. I wouldn't call them great movies though.
Still, the setup for the cross over, which the actual comic story arc progression has happen pretty soon with Thanos being involved means the Avengers and Fantastic Four should be working together pretty damn soon.
But that would also have to involve the X-Men. Which would have to involve that shitting movie franchise. I'm sorry, but the X-men movies changed SOOO much from the comics that they don't fit the stories anymore. That was annoying as fuck to watch. They were alright as movies, but overall I wasn't happy with them.
Which is why I LOVE the avengers right now. They've stayed pretty damn true to the original stories thus far.