I sure don't see the movies as based on the Ultimates universe - Thor and Iron Man's origins have little to do with what's happened in the Ultimate Universe. Ultimate Cap is a bit of an arse - and the movie Cap is nothing like that...Ah that makes sense since this isn't the main Marvel Universe but based on the Ultimates universe. There is also a version of these bad guys in the Ultimates animated movies that Marvel put out.
There are strong ties to both Marvel Universes...I've always thought of the Marvel movies as being based more on the Ultimates Universe. Why else would they have Samuel L Jackson as Nick Fury?
The Thanos bit was also mentioned as a possibility along with the Vision bit.I've read other rumors describing a completely different post-credit sequence, involving Thanos. Maybe they're filming multiple post-credit scenes, like they did with Wolverine: Origins?
Or maybe these rumors are all bull####.