Amongst the Kingdom Hearts III information revealed at the Kingdom Hearts 15th Anniversary Commemorative Campaign Preview Night held at Square Enix Café Osaka on October 13th, 2017, there was also some discussion pertaining the worlds in the upcoming title and future reveals. Kingdom Hearts III development leads, such as series director Tetsuya Nomura and lead character modeler Mr. Yamasaki, answered related questions.
It was confirmed that a new playable character other than Sora will be present in Kingdom Hearts III. They said:
Such a world will come out [where this is possible]... But it is something we cannot tell you right now.
This follows a previous statement where Nomura hinted at the possibility of a second playable character.
When asked about the world selection criteria in Kingdom Hearts III, the staff in attendance answered:
Although we put emphasis on the play-element of worlds, sometimes we like for the new worlds to be a surprise.
When asked if they meet with Disney representatives abroad to discuss world selection, Yamasaki replied that he does so on a regular basis to share his point of view with the world, and that "each leader goes [abroad] as a delegate."
Lastly, the staff stated that while D23 Expo has been confirmed in the past to present new information on Kingdom Hearts III, it is undecided whether this will be the case for the event's upcoming 2018 installation.
You can read more about the Kingdom Hearts 15th Anniversary Commemorative Campaign Preview Night and view pictures of it here.
The Kingdom Hearts 15th Anniversary Commemorative Campaign Preview Night at Square Enix Café Osaka is a campaign that precedes the joint campaign between Square Enix Café Osaka, Square Enix Café Tokyo, and ARTNIA, which is held to celebrate the Kingdom Hearts 15th Anniversary as well. You can read more about this joint campaign and view pictures of it here. This collective campaign will run at ARTNIA until December 14th, 2017, and at both Square Enix Café Osaka and Square Enix Café Tokyo until November 10th, 2017.
You can also keep in touch with the ongoing updates on our Twitter here.