I think it would be based off Kingdom Hearts Unchained Key/Union Cross but with Kingdom Hearts 3 graphics and mostly the same gameplay as Kingdom Hearts 3.
Medals could be incorporated into the gameplay, just that certain medals would give you different abilities and situation commands.
You would still get a Spirit like you do in KHuX but it'll be a party member.
You would get several voices for your character when making your character in character creation. You can obtain new outfits and accessories from a Moogle as well battle items using Munny (that you earn in game, no Microtranscation stuff).
The main hub world would be Daybreak Town, they don't necessarily have to add new worlds, they can use the ones from the base game of Kingdom Hearts 3. The book of prophecies can see into the future so why can it not see San Fransokyo or Arendelle? The worlds would feature a story from their movies. Big Hero 6 will be the movie plot and probably the same with Toy Story but with heartless added as filler and Chrithy commentating on stuff at certain points in the story.
If they do add new worlds it should just be the ones that we see in KhUx like Wonderland and Agrabah. If they add the worlds from KhUx, I hope they fix the story progression a bit because the worlds are too slow and they should be done in one visit or two. The new worlds wouldn't be as big as KH3's worlds though.