I agree. I think after all the complaints about no FF characters in 3, to putting them in ReMind, and then to not include anyone in 4 would not be the best look, so I'm inclined to think Nomura would throw someone pre-FFXV in somewhere (Noctis and co, and Clive won't be in a KH game).
It's funny that it's the characters themselves that make KH fans feel like FF is included. While I did miss the characters like Leon, I didn't feel a Final Fantasy presence was lacking in 3. I mean, you build a giant Cactuar to climb in Toy Box, you unlocked FF Gummi Ships, you took photos of constellations of FF monsters, the Dissidia display... I felt very happy as a FF fan, but I recognise that I'm not a majority voice in this.
But yeah, they're pretty good with taking feedback and executing it in their next game so I do feel like they'll be putting a FF character into KH4.... hopefully.