Legally, no. And even if MS owned FF, why would the Avengers want to team up? FF are a bunch of losers compared to the Avengers. Who needs a guy who uses flame when you've got a god who controls lightning? Who needs a strong guy when you've got the strongest guy? Who needs a rubber man to reach shit when you've got Iron Man flying around? Who needs an invisible woman when you've got the swag of the Avengers?
Haha, finally someone who thinks the F4 suck as much as I do.