Kingdom Hearts 3 ended on the biggest cliffhanger, as many questioned what would become of Sora and all that. There have been hints, of course, but fans do know he's been sent to a city based on Shibuya, otherwise known as Quadratum. Answers could still be quite a ways away, but fans have gotten their first official look at the next Kingdom Hearts game.
During the Kingdom Hearts 20th anniversary event, a new trailer debuted confirming two new games in the Kingdom Hearts franchise. The first is a mobile game called Missing Link. The second is a real shock, given how far and few between the mainline titles are, as it the new mainline entry simply titled Kingdom Hearts 4. According to Square Enix's press release, this is the beginning of a new "Lost Master Arc" for the series.
It sees Sora waking up in Quadratum, which a young woman describes as both a city full of life and an afterworld for those like them, and having to face a giant Heartless there. It seems 7 days have passed overall, since his arrival. The Kingdom Hearts 4 trailer also opens by saying that if someone isn't happy with an ending, then they can leave this world for another; however, it also ends by saying if they leave this world, then they can't expect to return to the one they came from. The relevant portion of the trailer begins around the 4:00 time mark; the subtitles are in English.
There is a brief glimpse of gameplay of Sora zipping up to the massive heartless, running up buildings, and more fancy footwork. It shows him seemingly deflecting an attack, while also using big, almost drill-like moves with his iconic Keyblade. It's not much, but it's certainly exciting with the newly updated graphics. This is subject to change, of course, as fans should also note that no release window or date was given.
Donald and Goofy appear after the Kingdom Hearts 4 title is revealed, and it seems like they are perhaps looking for someone who can "help." A blue flame is seen, but the identity of whom they are looking for is not confirmed by the trailer. Presumably, however, whoever it is may help them find and/or look for Sora following Kingdom Hearts 3's ending.
Kingdom Hearts 4 is in development.