People tend to overthink the villain thing. The reason people came to see the Avengers, and will continue to do so, is to see the heroes, not the villains. Nobody went to see the Avengers to see the Chitauri, or even Loki; they went to see their heroes. Same is largely true of the solo films....nobody gave a crap that Ironmonger and Whiplash + Hammer were ****ty villains; they *still* went to see Iron Man do his thing.
Yeah, but we have seen them in a team up, and will so again... and again... and again.
So what is the draw after we have gotten a couple of movies like this? I think the franchise will bank regardless as long as it retains this cast and director, but not without limiting returns here on out. Iron Man 3 is featuring his main villain, meaning I wouldn't bank on an IM4 anytime soon. That's telling me they are at the pinnacle with that character right now and probably aren't going to risk waiting around and dragging things out with secondary villains. We still have some good IM villains that will be unexplored for some time if that's the case.
I think the pinnacle for an Avengers movie are still Hulk and Iron Man in major featured roles. Those are the characters I think the audience would love to come back for at least for one more film before it starts getting redundant. Meaning you will make a billion dollars plus even if you put a Transformers quality sequel out there. After that, you are going to need top notch story telling and direction, because the team up thing will wear out its welcome and secondary heroes will be more heavily featured in place of the original roster.