I don't think the idea is a good concept to start with and I feel many people have touched upon reasons that I agree with for the most part. TWEWY is a good standalone game I hope SE never makes a second one because I don't want the game to be killed as I can't see Neku returning as he has already gone through the game 3 times.
On the topic of TWEWY and KH mixing together I just don't see it working as the game have two completely different art styles that wouldn't blend well as TWEWY has thicker borders to their characters and is in 2D compared to KH's more detailed characters in a 3D world. Besides that, the games don't really make sense if they were to combine.
TWEWY deals with someone dying and getting a second chance at life almost, and that is something that Disney would not want KH going towards as it deals with death. In order for Sora and friends to meet the characters from TWEWY that would require their death, and Disney does not deal with death well, lets just put it that way.
Not only that, but TWEWY is about a boy in this world which he can interact with, but he can't really affect the world that much as he doesn't interact with the Shibuya people besides the couple people in the game. For example, the game Bayonetta deals with a witch in this world she can interact with, but she can't directly interact with the people in the world as she is in a different dimension. It is like that with the TWEWY and that completely goes against KH, which is about a boy who affects every world is goes to by getting involved. The two games wouldn't fit if they were to come together as one is about a boy who is unseen by those around him compared to a game about a boy who interacts with everyone.
