Giacchino would do a great job, in his short few years scoring movies (less than a decade so far) he's joined the leagues of Williams, Goldsmith, Silvestri, and Horner. I'm rooting for Silvestri to return because I really enjoyed what he did the first time around, and would like some continuity. The guy's music just sounds
like a Jack Kirby comic to me. I also think now that The Avengers are an established entity, he could expand on the thematic work he established the first time around and try some new stuff.
But if neither of them could do the job? David Newman. A ridiculously overlooked composer, who knows his way around a hero theme and can be absurdly inventive with his instrumentation under the right director. He also has experience working with Joss, having given him the best score of his whole career for Serenity.
Take a listen to his score from The Phantom: