Keybladewielder on 16 July 2017
So Nomura has hinted that another character will be playable aside from Sora and explained why more than 2 guest party members (up to five) can join Sora, Donald and Goofy.
http://www.ign.com/articles/2017/07/16/kingdom-hearts-3-director-hints-at-second-playable-character
I wonder who the other playable character will be. Maybe Kairi during the tutorial, or Riku since he's having his own separate adventure with Mickey or maybe they'll pull a KH2 and have Roxas playable again (though obviously it wouldn't be at the start of the game like in KH2).
Edit: Also, more info on the Toy Story world
