I dunno', don't get me wrong I'd love to see it work out. But whenever I think Marvel Cosmic-U the whole thing steers into DC's 'Green Lantern' territory. As in 'Hot Wheels' and 'juvenille' blah-blah. It was pretty, but it all seemed like a waste of a Ryan Reynolds (where the fuck is the Deadpool movie? That would be

, give it to us already!!)
The cosmic dimension of Marvels story-telling universe is so deep, so rich and colorful, introducing it slowly and not shoe-horning it in just to make an 'Avengers 2' timeline/schedule, makes more sense. A Thanos story, if done hastily, has the potential to be a little 'too big for its britches' in a way, too much too soon. Could there be enough intro-space in the next series of solo films to make a Thanos-centric Avengers 2 make sense to people? I dunno', it sounds like a stretch.
I'm not trying to be a killjoy, but I was a huge 'Heroes' fan. And what they did to season 2 of that show has made me aware that the greatest things can turn to utter crap on the follow-up run.
I also mention it because many of the credit-scenes of the Marvel films have hinted at plot points to the Avengers. Perhaps the Avengers credit-scene was meant to point to plot points in a different film, Thor 2 perhaps? Iron Man 3? I have no idea, Im just talking out of my ass here, but it seems a bit early to be teasing for the sequel when there is a lot of time, or at least ground to cover before that happens.
While I know who Thanos is and you know who Thanos is and a percentage of people will google wtf a Thanos is, the other 10 million people who went to see that movie this week might not. Will they 'get that' cosmic-dimension of the Marvel U by the time an Avengers 2 is in the works? Or wouldn't you think that Thanos' post Avengers appearance is a tease for something coming much sooner than an Avengers sequel? I do, but I'm insane sometimes.
Plus, Whedon has stated that (if he directs the sequel, and considering the number$ already they'd be on crack not to re-hire him since I read that was already on the table or a done deal or something) he wanted a sequel to be 'smaller...more intimate' sort of thing. Yeah, that doesn't sound anything like a cosmic war featuring an infinity gauntlet. Just putting two and two together folks. My guess is Thanos is the big-bad in something else.
Also, considering that weekend box-office, some ungodly number come Monday I'm sure, Disney would be insane to not just go ahead and spend the half billion it would take to buy back the rights to every Marvel character right now. Fantastic Four, X-Everything, Spidey, all of it. Since the first team outing has just made a half billion dollars this weekend, could a 'team-up' happen in the future? Could Cap travel to Asgard, have a major cameo in a Thor film? That sort of thing?
Edit: btw, what I have stated above is purely speculative BS and I will only lazily defend any of it. I say this because of the complete shock I am still in at how awesomely, in the grand and enormous sense, the Marvel U was presented in the Avengers. I could be completely, and pleasantly surprised that it all works out next time, regardless of what I think the chances of x, y, or z being 'plausible' going in are. Frankly, I'm expecting to be surprised again.