Let me start by showing you the chart of all the rights for Marvel characters in movies:
So, this really helps one answer the question: Given the current movie rights, who CAN be on the Avengers?
Well, the better question would be... Who cant?
Sub-Mariner
Man-Thing
Spider-Man (And all his villains, minus Iron Patriot). It should be noted Sony is currently in talks With Marvel to merge.
Fantastic Four (And all their villains, minus the Kree)
Wolverine (And Sabertooth)
X-Men (And all their enemies, minus Quicksilver + Scarlet Witch)
Cable (And... Well, he's below)
Deadpool (He's made too many enemies to know who owns who)
Silver Surfer
Everyone else, Marvel Studios can use!
Any questions? Anything you want me to clarify? Ask!