This is a nice topic, I'll let it stay open to see other's ideas.
Also, mines would be
"The Excalibur Key" . It's like a fusion of both Sora's and Mickey's keyblades(Since they are like a cross between each other anyway), and increases the users skills 10x as much. The real version of the Keyblade
All those who show true wisdom, will live to see the wonders of the Kingdom, Hale to the King Of Games. Join Team "Royal Kingdom$$$$"