Actually you did pretty much sum up what they plan to do with Captain America for Avengers. The entire movie takes place during WW2 and they most likely will freeze him until he's needed.
I'm also glad to see Loki will be back, as the actor playing him fit the part rather well.
As for the Avengers movie, we have Iron Man, Cap, Hulk and Thor, along with a few other parts yet to be announced. And as Afternoon said, another Avenger was in the Thor movie as well. Hawkeye, the dude on the lift that was supposed to take out Thor with the bow. And then of course, we also got to see Black Widow from Iron Man 2. So that makes 6 Avengers, not counting the main man, Nick Fury.
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