At a recently held Kingdom Hearts Union X developer panel, series director Tetsuya Nomura teased that a new character and respective world could be coming to Kingdom Hearts 3 - and that the news would be official relatively soon.
It's possible this new character could come in the form of a summon, but who that character is and what world they're from Nomura was tight-lipped about, as one would expect. He did say, however, to keep an eye out for this new character in a new trailer coming... soon? No details on when that would be happening either, but I think we can expect it around E3, possibly sooner.
Kingdom Hearts 3 is still set for launch sometime in 2018 for PS4 and Xbox One, though no firm dates ave been given at this time. Its likely we'll get that information at E3 as well.