I see it as...
Rainbow Dash as Iron Man,
Twilight Sparkle as Captain America,
Fluttershy as Hulk,
Applejack as Thor,
Rarity as Black Widow,
Pinkie Pie as Hawkeye,
and Princess Celestia as Nick Fury.
Starring:...
Iron Mane,
Captain Amareica,
FlutterHulk,
AppleThor,
Black Rarity (No racism intended),
Pinkie-Eye,
and Celestia Fury.