Mako: Batmako. Parents killed by random criminals when he was a boy, broods a lot.
Bolin: Super-Bolin. Also has no parents, is upbeat and cheerful, physically powerful, and has the right hair for it.
Tenzin: Martian Manhunter. Caped intelligent man, is immensely powerful but not in an obvious way.
Amon: Doctor Doom. Easy.
Korra: Wonder Woman. Trained all her life in fighting arts, is almost superhumanly strong, traveled across the sea to the normal world, and has a mythological origin.
General Iroh/Asami: Tied for Iron Man. Asami because she's a genius billionaire playgirl philanthropist whose father built a company she's trying to demilitarize, Iroh because he flies on jets of fire and because it's just too easy to call him Iroh Man.
And last but not least, Lin Bei-Fong is the Spectacular Spider-Lin: Immensely strong, intelligent, has a special "sense", and she uses a device to send out tendrils from her wrist which she can use to swing across the city, tie people up, or smack the crap out of criminals with.
First off, this is genius and made me laugh so much. May I ask, how did you do the text? If you photomanipulated the words from an Iron Man poster, you did a flawless job. If you generated the text and textured it yourself, what, I didn't even know such things were possible.