You can submit stand-alone attractions, Marvel lands, or, if you're really ambitious, an entire Marvel park.
Here are the rules, though: You can't suggest anything Disney can't build. That means no proposals for Walt Disney World or Tokyo Disney. Proposals also have to go onto existing Disney property, so no expanding the existing property boundaries in Anaheim, Paris, or Hong Kong (though you can feel free to get as creative as you wish in cramming stuff within them). If your idea wouldn't fit in existing free space, you've got to declare where it would go and what it would replace.
I was thinking about a Marvel Land at Disney's Third Gate Park.
* A 4D Immerse Tunnel attraction named Hulk 360-3D, where you can ride the tram, put your 3D Glasses and watch Hulk battle the Abominable.
* The Avengers: Battle for Earth, A 4D Motion Simulator similar to The Amazing Adventure of Spider-Man.
* Flying with Spidey, A Motion Simulator ride similar to Star Tours.
* Iron Man Experience, A Flying Motion Simulator ride similar to Soarin', not a clone of an Upcoming Hong Kong Disneyland attraction of the same name.
A few ideas:
-Shield mission ride where guests become shield agents in a black (open top) van (motion base vehicle or coaster car) through some epic Shield adventure where something terribly goes wrong (iron man needs help or maybe characters from the TV show require saving)
-Helicopter Ride (motion simulator) through some big alien invasion
-Iron Man Shooter
-Asgard Walkthrough or an attraction on some Asgardian vehicle like seen in Thor: The Dark World.
-WDS-behind the backlot tour to complete the loop (helping the currently awkward design
-New HK park or in expanded Tomorrowland
-Replacement for Bugs land or parts of Tomorrowland in Disneyland (california)
-Stark Expo show where something goes wrong and iron man and heroes come in to save you, BUT the seating area can "fall apart" aka shift or tilt when a support breaks or when a villain enacts some malicious force. Tony and gang of course save the day and hulk or someone to resets the seating area
General: I don't think a single Marvel attraction is enough given the vast amount of substance in the brand, but I also think building a full park themed to Marvel would be a misstep as those not interested in superheroes would be likely to pass it up. Therefore, both of these proposals are for new areas. Also, since the Marvel Cinematic Universe is the big Marvel franchise at the moment, the attractions are primarily based on that.
California: Marvel land could go into either park without too much hassle, though it would be better suited for Disney California Adventure Park.
DCA: Hollywood Land would be rethemed as New York City. The Animation building would remain, but Marvel elements would be incorporated in addition to classic Disney animation. All other attractions with the exception of the Twilight Zone Tower of Terror would be removed. New attractions include an indoor flying coaster themed to Iron Man (replacing MuppetVision and the Millionaire building), a S.H.I.E.L.D. themed dark ride (replacing Monsters, Inc.), a walkthrough of the nine realms inside the Mad Arcade building, some type of Captain America attraction in the Disney Jr. space, and an Avergers stage show in the Hyperion Theater. The main restaurant of the area would be a Shawarma restaurant in the area occupied by Award Wieners and its seating area. The shops would remain but receive a touch-up. This would be an alternative to the Monsters, Inc. area.
Disneyland: New York City would replace Toontown and the surrounding backstage areas. It would feature the same attractions as the DCA proposal (minus the existing attractions), but in a different orientation. Nothing from Toontown would remain.
Paris: While a Marvel area would probably fit in Disneyland Park, it would be better used to revive Walt Disney Studios Park. The Studio Tram Tour would be removed and the entire area used for a Marvel Universe. Unlike the California version, there would be several sub-areas (Los Angeles, New York, S.H.I.E.L.D. headquarters, Asgard, etc.), and each would feature attractions themed to the appropriate characters. Also, this proposal would not be restricted to the Marvel Cinematic Universe, so Spider-Man and X-Men attractions would likely be present in addition to Iron Man, Captain America, Thor, and Hulk.
On your note about having an entire park devoted to Marvel....
I think the "Heroes & Villains & Adventures" style concept would work best. I think someone has mentioned it here before, but basically it was an idea for a new Disney park.
It would include lands based on Marvel, Star Wars, Disney villains, etc.
Walt Disney World
Tokyo Disney Resort