I also think they need to open up a lot of the MCU cosmic side through The Avengers, as well as other aspects of the marvel universe. We could get an intro to a few characters related to Thanos. There were rumors at one point, of a Thanos solo movie(with him as an almost anti-hero) and a GOTG movie; so they may be setting up Thanos, for his own film, or Avengers 2, or another cosmic marvel film. Maybe multiple. He could become an even more bad ass overarching threat than Loki.
I'm thinking Thanos will be supplying the mercs too. Staying offworld somewhere in the universe on his ship. He might be a little bit more than a cameo though, and could be tied into the post-credit scene as well like lots of people are saying. The point I'm making is that there's so much going on in this movie that I expect Thanos' screentime to be short but effective like the emperor in Star Wars. It really seems Loki will be our villain for the majority of the film. We'll probably see him controlling the Hulk as a major plot point still. So, there's just too much going on with the invasion and formation of the team, and Loki possibly switching sides--to have an all out battle against a fully powered Thanos. Maybe some lesser characters working for Thanos.. And if Thanos does show up Loki's not going to let him just take the gauntlet with all the gems. I could see him scattering them across the 9 realms so Thanos has to spend time across the movies gathering them. Then they would only be fighting Thanos pre-gauntlet and gems, and could have Loki's help. Or, maybe he only hands over the gauntlet fulfilling that part of the bargain.
It would be cool just to see Loki make a quick deal with him to get the army. It really seems like the gems/gauntlet are involved, but I think they will make Thanos' quest span a few movies to turn Thanos into the ultimate threat. I just can't see a powered up Thanos appearing in the first film. If the rumors are true, I expect Loki to doublecross Thanos or join up with the Avengers against his forces. Thanos could just be after the cosmic cube too and the gauntlet may not be involved at this point. Maybe he'll get one gem, or the gauntlet, or somehow gains either in the post-credits.
If Thanos appears on earth and confronts the avengers then I really think he won't have all of the infinity gems. It could be that by the end Loki has no intention to hand over the gauntlet/gems or even cosmic cube at all.
