Iron Man is probably the most popular Marvel character right next to Spider-Man. Whether he's on screen or not currently, it would be smart to capitalize on him most, imo. The issue is, if you're going to do a Iron Man ride, it really should be a flying coaster like the rumored Iron Legion coaster, but IOA's last two additions will have both been coasters, with 4 total in the park after JP. I can't see them adding another coaster, especially to a land that already has one.
Something I think potentially could be cool would be if they used the Toon Lagoon Amphitheater to do a Marvel stunt show. Marvel has more than enough characters in their arsenal to make something like that work and it would be way more appealing than something like Sindbad was.