On the note of fleshing out worlds, I Was going to say that a long while back... or maybe I did? I have memory issues. But basically what I wanted to say is...
For me personally it's important that it feels like the worlds matter. It's not just a place to show up and do 'Kingdom Hearts' stuff. But it's also not just a place to show up and do 'Stuff from that world'. The best way I can explain it is...
In KH3 I really liked the Toy Story world, because you set off on a completely unique adventure separate from the movies, the characters all interacted with Sora, Donald, and Goofy, and cared that they were there, and their presence had a legitimate effect on the story, and in fact the way Buzz was so suspicious of them, the fact they didn't belong in that world pretty much drove the plot.
In KH3 I really disliked the Frozen world, because it literally did not matter if you were there or not. It was just, the events of the movie happen, and meanwhile Sora, Donald, and Goofy also happen to be there and kinda sorta help Anna out with something but beyond that, the events of the movie transpire completely separate from your own story, it just happens to be in the same world while Sora chases Larxene around.
There is no point in going to a world just so that the events of the movie or other media from that world can take place while our characters do their own separate thing in the meantime.
That's why I also loved San Fransokyo. It really felt like everything that happened in that world could be used to set up a sequel.
Also, I don't get why people were suddenly concerned with interfering in the happenings of the world. Why didn't anyone call out Sora for meddling in Tarzan's story as much as he did or Alice's if it was such a big deal?