Just throwing in a few ideas, a few of which were obviously inspired by others' ideas.
Muppets - Playable characters can include Kermit, Fozzie, Miss Piggy, and Gonzo. You could create your own shows/films by playing mini games on stage! And perhaps there could be a specific story that involves exploring the studio and/or Hollywood.
Gravity Falls - Playable characters can include Dipper, Mabel, Gruncle Stan, Wendy, and Soos. You explore the town and forests of Gravity Falls and solve mysteries involving paranormal activity. Potential enemies can include the rainbow-puking gnomes and living wax statues. And I totally agree with using Waddles as a mount! XD
Wall E - With Wall-E and EVE as playable characters. Perhaps the main objective of the game is to make the earth more habitable for mankind. Missions could involve destroying garbage, salvaging useful items, finding places to plant seeds and grow plants, and maybe even fight rogue robots for the sake of more action.
Spirited Away - With Chihiro, Haku, and No Face as playable characters. The game's objective could be to serve the various spirits in the bathhouse and visit the colorful ghost town for a few missions, while at the same time finding a way to escape from Yubaba's servitude.
Maleficent - This movie won't be out for a while, and we know little about it. But it sounds like such a cool idea! http://en.wikipedia.org/wiki/Maleficent_(film)