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Fire Emblem with Creative Assembly - The Fire Emblem series has ties to the Strategy and RPG genres, but this collaboration would focus more on the strategy. Creative Assembly has a fairly solid record making turn-based/real-time hybrid Strategy games despite the problems of Rome II, and I figure that the large casts of characters in Fire Emblem could easily translate into leaders of larger armies. The scale could be a bit smaller than typical Total War game, with battles of hundreds of troops rather than thousands, instead focusing on the interactions and development of individual characters. They're already mixing things up with Warhammer, why not throw in shapeshifting dragon people?
Theatrhythm Zelda with Indies Zero - Indies Zero is the developer of the Theatrhythm games alongside Square Enix. So far, that series has tackled Final Fantasy and Dragon Quest. Thing is, Curtain Call ended up having over 220 songs from the Final Fantasy series. The only other gaming franchise that deserves a title celebrating its music is The Legend of Zelda. It couldn't be a direct copy of the existing Theatrhythm games, but this is the logical next step for Theatrhythm.
