Easily the most notable thing to come out of D23 was the confirmation of another Pixar world in Kingdom Hearts 3- Monster's Inc.
Unfortunately, this doesn't really come as much of a surprise as leaks during the past few months already revealed to us most of what we saw of the new world, although there are significant graphical changes to Sora, Donald and Goofy's monster designs.
The world features the trio in the Monster's Incorporated building with Boo, Sulley and Mike, although whether the visit takes place during the film or after is entirely up for speculation, as the whole facility looks a bit abandoned. Whether Square Enix would potentially change the ending for Monster's Inc to fit Kingdom Hearts 3 is another question entirely.
One of the main features of the world appears to be the doors, similarly to how they functioned in the actual Monster's Inc film, although whether they act as transports to other worlds isn't seen in the trailer.
All of this makes the Monsters Inc world one of the most exciting to appear in Kingdom Hearts yet.