So I've seen people speculate that Riku's new Keyblade could possibly be that of the Realm Between just like how Kingdom Key (L) and Keydom Key D represent the realms of Light and Darkness respectively.
Well, today I happened to come across the old KH II secret ending again. In it, Terra picks up KK, Aqua KKD, and Ventus picks up Way to Dawn. Now, sure Way to Dawn at the time made sense as it represented a middle road. But what if that scene inspired Nomura to create a true counterpart to KK and KKD representing the Realm in Between.
One hint that Riku's new keyblade could have some connection to the Kingdom Keys is the Mickey Keychain which has thus far only appeared on the Kingdom Keys.