I think i agree with most that the scores for Marvel movies have thusfar been somewhat lackluster. I like big themes and memorable melodies as much as the next guy. I did like what Armstrong did for TIH however, very Bourne like in places, and he did try hard to work in some thematic material for the Hulk even if it wasnt something you might whistle to yourself. Though ive never really liked/recalled anything Doyle has done before i did like what he did for Thor and he came up with a very nice Thor/Asgard theme IMO. So I guess its really the Iron Man movies that I am dissapointe with. I suspect that might have a lot to do with Favreau's keenness for the rock guitar sound - I would have thought something more electronic would have been a more natural association for a tech based character like Stark.
Anyway it seems a budget conscious studio like Marvel are more likely to go with a cheaper, smaller name composer for an aspect of the film that they probably see as largely interchangable like the score. I hope this changes for The Avengers however (the decision to go with ILM as the main effects house might indicate a more "top shelf" approach) or at least that a lesser known guy can come in and knock it out of the park.
I can see there is a fair bit of antipathy towards Zimmer and his crew (not uncommonly amongst score afficiandos) but im a fan and i think the Joker's theme from TDK was perfect. It may be "one note" but the sound design was awesome and totally suited such an anarchic depiction of the character. I would love to see him given the gig.
Giacchino would also be great. Elfman is fine but i think that would be a somewhat boring choice. I really like Jackman's score for First Class especially from someone id never heard of before. Powell's X3 score was great and in some parts was phenomenal (Dark Phoenix's Tragedy) and I think he would be potentially epic. Im sure we will end up with someone nobody saw coming however