Agree with Applejack, Pinkie, and Rarity's roles. As for the rest, these roles better suit them:
Twilight - Thor Rainbow Dash - Hawkeye Fluttershy - The Hulk (note that this was BEFORE the Power Ponies episode and instead based on Putting Your Hoof Down) Spike - Agent Coulson
This is awesome. Applejack and her wholesome all americanness is just soooo Cap. Dunno if Pinky the Partygirl is right for Ironman or Rarity (Who would make a zillion costumes) would be better, leaving Pinky as Black Widow (especially since Rarity might see basic black as too dull)... but Dash as Thor? Perfect.
