Sora should be something of an end goal of whatever the next game is, not so much the main character. I think once upon a time I said that the game after KH3 should definitely be the "Search for Spock" of KH, and it really has ended up being just that. So they need to lay all in on that.
Have Riku finally go on his own real adventure to different worlds. I love the idea of them being worlds primarily focused on death as the theme. Perhaps it would be important to learn that what is lost isn't always gone. Maybe he needs to learn this just as much as Sora that sometimes you can't save everybody, but rather you have to learn how to face what is in front of you head-on and find the truth.
After all, Sora and Riku have both ended up doing the same thing. Riku died to try and protect Sora, and Sora died as a result of saving everybody and Kairi. They give too much of themselves and they always inch closer to the point of no return.
And maybe all of the worlds don't need to be full on death-themed, but I think stories that focus heavily on themes of change and letting go can also fit. Frozen 2 is a good one imo, so would Ralph Breaks the Internet, Toy Story 4 just to name a few modern ones.
I don't really imagine KH will be changing the formula for how worlds work, except maybe instead of traveling to each world there are portals.
On the other hand, this bridge game is the perfect time to experiment again before KH4. They could explore the series without a heavy Disney influence and focus on pure story development.