It appears as if Square Enix is focusing on quality over quantity with Kingdom Hearts 3. This is because it might have less worlds than in Kingdom Hearts 2.
This news was confirmed recently when Famitsu interviewed Tetsuya Nomura recently. Part of the interview was translated/summarized kindly by a user on NeoGAF.
Anyway, Nomura specifically said that the number of worlds in Kingdom Hearts 3 will be smaller than that in Kingdom Hearts 2. However, the density of the worlds are much bigger and grander this time around.
To put this into perspective, Kingdom Hearts 2 had 12 worlds and the first game had 9 worlds. It’s possible the third game could also have around 9 or 10 worlds.
An interesting part is that he said that the Toy Story world takes place after the events of Toy Story 2. Not to mention he also said that the events in the game is actually canon to the Toy Story universe. Lastly, he said the Big Hero 6 world in the game is coming along nicely but they are not going to share it with us until a later date.