Originally Posted by I'm Venom
You've missed a second option, which I've seen some rumors to support:
Use Thanos as the villain in Avengers 2, and the Gauntlet is one of many reasons to favor Avengers 2 over Avengers 3.
Yes, it's possible the Masters of Evil could appear in Avengers 2, but you're looking at turing an ensemble film into a double ensemble film. There's a risk of it turning into another Expendables - having to spend so much time developing each character that it turns into a mishmosh. That'll translate to box office problems.
It only works if you treat the group it self as a Character. So you would be telling the story of the group instead of trying to develop each individuality character.