My dream video game for Disneyland is an MMORPG
called "Knights of the Magic Kingdom."
The premise is that there is an alternate dimension connected to Disneyland where the real Disney characters live, in "homelands" that branch off of the corresponding rides. This alternate dimension is the manifestation of the excess creative energy generated by Walt Disney Studios when they make movies, and various spiritual entities come to feed on it. Some of them are harmless, but others are quite dangerous, and not only do they pose a threat to residents of the alternate dimension, but the damage they do can reverberate back into the "real world" and cause problems at Disneyland.
To cope with this, those in the know at Disney secretly recruit brave, sympathetic people and teach them how to cross over into the alternate dimension and use the magic there to devise empowered identities for themselves, so that they can fight the monsters and other weird things. These people are the Knights of the Magic Kingdom, AKA the player-characters.
At character creation, you would choose an identity and powers for your Knight based on one of the lands/attractions at the park. For instance, you could be a pirate with mad swashbuckling skills, or a fairy with transformation magic, or a Toon with nigh-invulnerability and infinitely deep pockets, or even a wild jungle animal. Each Knight's powers would be slightly more effective in their corresponding land than elsewhere.
Missions could range from retrieving a small child who accidentally crossed over into the alternate dimension, to helping Peter Pan rescue the Lost Boys from Captain Hook, to taking down a monster that threatens to destroy the analogue of Sleeping Beauty Castle, which would almost certainly have catastrophic consequences for the park in the "real" world. The game's currency would of course be Disney Dollars, but some shops might only accept doubloons or simoleons or shillings, necessitating a trip to the Bank of Main Street to make an exchange. Another general challenge would be accomplishing certain tasks in Disneyland itself without accidentally displaying your powers in front of NPCs (guests).