Kingdom Hearts 3 co-director Tai Yasue has discussed some of the changes fans can expect when they get their hands on the game in a recent interview.
While the game will strike out in a new direction, "we don't want to change what is fundamental about Kingdom Hearts. We're hiring new people continuously so they are learning about Kingdom Hearts through making 2.5 (HD Remix). It's a good learning experience."
Among other differences, Square Enix is introducing key blade transformations, attraction flow rides, and Sora is going to be more agile in the game.
Another notable point is that the Kingdom Hearts 2.5 development team are learning about the franchise through their work and will carry that over to making Kingdom Hearts 3.
It was recently suggested that Kingdom Hearts X Heartless could appear in Kingdom Hearts 3 and we got a recent look at how the game's weapons and key blades may look.