This seems pretty simple to me and it'd actually be easy to tie them all together before the Avengers movie.
After the end credits of Iron Man 2, Iron man meets Thor somehow(the hows and whys can be worked out later) or another and it ends.
Thor's movie begins and ends up at the same place Iron Man 2 ended, i.e., Thor deals with whatever he deals with during his movie and after his closing credits, we get the SAME post credits scene from IM2, only this time it's from THOR'S perspective instead of Stark's and it ends.
Captain America, SAME THING, only from Cap's perspective. I will say that during IM2 and Thor's version, Cap might have to be in the shadows or difficult to make out if they haven't cast Captain America yet.
The Avengers film will open with this scene being fully realized and much longer. As for what Avengers should be about? I think the main crux of the film's action would be Avengers vs. Hulk as that would be the main reason they are assembled. Subplots would include Captain America adjusting to our time as well as Iron man butting heads with Cap about an number of issues including leadership. Also, i'd throw in at least 2 more Avengers there. Possibly 3. I'd use Hank Pym, Wasp, and Hawkeye
