Thanos for The Avengers movie?
If he does appear then it should be in a sequel.
As diesel as those pictures are, they are not a prop for any movie. It was just a replica of the Guantlet for display at Comic Con.
I know this because my friend was all over that thing at Comic Con, and thought that it was for one of the movies, but they found out that it was just a display thing. Nothing more to it.
As for a villain, I wouldnt be suprised if they went the Ultron rout. Or go even crazier and have Loki show up with the Red Skull.
Plus, if they had Black Panther apart of the cast, I'm certain Joss Wheadon would've gotten Chiwetel Ejiofor to play T'Challa. I think that guy was born to play that part.