The keyholes shouldn't be in some random place. The keyholes are essentially the very core of the worlds. The world's heart. There has to have meaning in the locations. Even some of the keyholes in KH1 didn't make sense. Halloween Town's keyhole...in just that plain sight? Agrabah's keyhole deep in the Cave of Wonders, completely secluded from the actual city? Atlantica's keyhole in Ariel's grotto rather than in Triton's throne room? (Though it would've given opening to Ursula...still doesn't make sense.)
I made a story of Kingdom hearts, and placing keyholes in different places in the worlds I covered. The idea of Agrabah's keyhole, hidden within the sea of sands, Halloween Town's keyhole placed in the tree clearing in the Hinterlands, connecting to not only Halloween Town, but Christmas Town and all the other holidays. I haven't gotten far to Olympus, Atlantica and Neverland, and Wonderland, Traverse Town, and Deep Jungle's keyhole were the same.
Now...due to how open KH3 is, a location for the Keyhole...is a bit tricky. As I said it would have to be in an appropriate place for the very Heart of the world. Toy Box's keyhole could perhaps be located somewhere in Andy's room, whether it being on the bed or his own lock on his room, the keyhole being there seems...suiting. Corona's keyhole could perhaps be resting behind the King and Queen's throne, just like the Land of Departure's keyhole, or in plain sight in the middle of the sun symbol in the Square, where you point the keyblade at the sun, and its radiance floats down to the keyhole like the lanterns.
Monstropolis....has had so much wasted potential. The very notion of doors and keyholes, and the very traverse of other worlds...it would've been a perfect thing to cover on top of Vanitas' plans to use the screams and fear of children. I'd say there should be...a door. A door constructed as the very first door...and perhaps that can work well as a location of Monstropolis' keyhole....because, I'm...mixed about using Boo's door as the keyhole. Twilight Town's keyhole will obviously prose the same as Neverland's keyhole, making its location on the clocktower. I'd say compared to Neverland's...it's the most fitting place for Twilight Town's keyhole.
The most difficult one for me to think of...is definitely Arendelle, the Caribbean, and San Fransokyo. Perhaps the Caribbean's keyhole would be a singular landmark located in the middle of the sea, or...perhaps the keyhole in Davy Jone's chest. Arendelle's keyhole has many...possible places it could be, because there are many different landmarks. On one hand we have the most plausible option being in the castle in Arendelle, or another plausible being the keyhole located in Elsa's castle.
The very heart of the world is...difficult for me to think on, in regards of San Fransokyo.
But that's just my two cents.