Well if not Thanos then some other major villain; Ultron ?:newwtf:
Ultron is a random ass pick....but he catches wreck breh. He's a major Avengers villain.
There's very few characters on the level of Thanos though, in terms of sheer
recognition when you see him....so virtually anybody would be
after the tease at the end of "The Avengers". Fred.

with Captain America's shield broken.


"Buffy and Angel?" but dude was basically G.R.R. Martin before "Game Of Thrones" was well known. He would kill off characters left and right....major characters, fan favorites, all that. So unless Marvel specifically tells him
I expect there to be deaths.