Kingdom Hearts series director Tetsuya Nomura recently spoke with the IGN staff about Kingdom Hearts III at D23 Expo 2017. While discussing the possibility of Kingdom Hearts III on Nintendo Switch and the rest of the series to go on Xbox, Nomura also discussed if there will be more playable characters in the future. Nomura says that it would be tough to introduce many playable characters, but he intends to have another playable character aside from Sora.
You can check out the full quote below.
"So it would be tough to introduce many, multiple playable characters, but there is an intention of adding a playable character aside from Sora. Unfortunately we’re not able to disclose who it is yet, but there will be [another playable character]."
Furthermore, Nomura discusses the reasoning behind multiple party members at once, confirming that this will be the replacement of D-Link from Kingdom Hearts Birth by Sleep. You can check out the quote below.
"D-Link, that has been done away with. It was actually a compromise in a previous iteration because we were not able to display multiple characters on that platform in that game. We’re able to depict multiple characters. Up to five people can join, even more, multiple characters can join your party."