Kingdom Hearts would be cool. Imagine the mane six as humans wielding keyblades. They could even bear titles like Hero of Magic, Hero of Kindness, Hero of Loyalty, etc.
They could even have different forms for their keyblades. The natural form of their keyblades would match up with their cutie marks, with their cutie marks even being on the keychains. They could also each have a keychain of what they got in season 4 for their key to the chest, and the keychain changes their keyblade's form to one that matches up with it. Like...
- Rarity - She got a spool of rainbow thread. Her keychain would picture that spool, and the keyblade would have a theme of fabric and sewing. Maybe the teeth of the key would be an open pair of scissors and a needle or something.
- Rainbow Dash - She got a Wonderbolts badge, so that'd be her keychain. The keyblade could have a Wonderbolts theme, and maybe the teeth would be a pair of wings and a lightning bolt.
- Pinkie Pie - She got the rubber chicken Boneless, so her keychain would have a little 2D rubber chicken. The keyblade would be party themed, maybe with some cheese mixed in (maybe the guard is a cheese sandwich with a big hole in the middle), and the teeth could be party hats.
- Fluttershy - She got a flower, so the keychain would be of that flower. The keyblade could be themed with a mix of flowers and the Breezies. The teeth of the key could be a Breezie wing.
- Applejack - She got a bit, so that'd be her keychain. The keyblade could be themed with gold, that bottle of Flim and Flam's elixir, and apples.
- Twilight Sparkle - She got Scorpan's necklace, so that'd be her keychain. The keyblade could have an alicorn magic theme, with some Scorpan and his necklace mixed in. The keyblade's teeth could be the pendant of Scorpan's necklace.