So as a decently bigfan ofboth Kingdom Hearts and the legend of Zelda I have of course wondered what a crossover between them would be like and here are my thoughts so far!
First off, I think the time for this to happen would be in Kingdom Hearts III, where the Kingdom of Hyrule replaces Thebes as the first world to visit for Sora to get the power of Waking.
I think when they first arrive, the Trio is going to be separated somehow, maybe by Ganon's Forces and then Sora is forced to fight off some of Ganon's Starderd minions mixed with the Heartless(Maybe Unversed? The feelings of Hatred in Hyrule would be a fun thing to have here.) Anyways. While fighting off a miniature Army, Sora gets overwhelmed and is saved by Link! This Link would be an Amalgam of sorts, but one who leans towards TP and SS in design, as I like that Design best.
There would be more lessons about how to be aproper hero from Link. and I think I would question Sora's motto of "my friends are my power." and ask if Sora is just a leech who doesn't provide anything on his own, making him question that and consider who he is as a Hero.
I think Link would also introduce the concept of the Triforce, and how to be in balance and proper, you needed to have all Three Courage, Wisdom, and Power. He might clock that Sora has the Courage and Power, but is lacking in Wisdom, and that's fine, everyone has differering amounts, but he needs to learn and develop that to really unlock his true power and potential. And we would have Sora coming to a realization that he gives everyone around him Courage. That is part of his power, to shine a light through darkness and provide hope.
Then I think in the world there would be a little dungeon section where you have to combine Link's bow with Sora's magic to do fun effects and puzzles.
Boss would be against Gannondorf, and Ganon would be his Heartlessified version.
I think the Keyblade you'd get from this would be called "Courage," the Base I think, would be reminiscent of the Hylian Shield, go into an actual replica of the master sword, and the key part itself would be a Triforce.
I think the transformations...I think first would be a Bow that fires quitly and you can chain it with casting spells to give cool effects to your arrows with the second transformation being a sword and board combo.