Not for the the faint of heart, you have been tasked by the powers that be to get a Marvel attraction in the major lands (We'll leave out New Orleans Square and Main Street USA for the sake of sanity, but they can be used if you want)
Tomorrowland: Easiest one "Guardians of the Galaxy" an immersive dark ride/thrill ride combo mixing elements of Indy and Radiator Springs Racers (I'm thinking an action packed dark ride using test track technology for an indoor high speed escape at the end.
Fantasyland: The Sword in the Stone is replaced w/ Mjolnir, Thor's hammer which can only be lifted by someone worthy of the power of Thor.
Frontierland: Revive the Western River Expedition concept using Marvel's western heroes characters
Adventureland: One of the tougher ones due to lack of space. Replace Alladin's oasis w/ a meet and greet/ restaurant based around Black Panther.
Critter Country: I think I stumped myself here
Toon Town: Honestly, if you give them a more comic booky look, some random Marvel characters could work fairly decent there.