There's no way they'll buy back Spider-Man, Wolverine, or anybody in any other other studios' Marvel movies. They'll simply be too expensive, especially considering they can just tell stories using others of the 5000 or characters Marvel has running around.
I can't imagine they'll go too much farther without introducing Hank Pym/Ant-Man, if not in his own movie, in
Avengers 2. Wasp too, as she's his wife - maybe like they did with War Machine, as 'herself' first, then she discovers his secret and adapts it for her own. They've already announced Mandarin for
Iron Man 2, but others to look out for are Ultron (for Iron Man or Ant-Man), Enchantress (for Thor), and Baron Zemo (for Captain America).
Also, spoiler from
Avengers: