The voice actress for young Kairi in both Japanese and English (Sumire Morohoshi and Ariel Winter, respectively) shares the voice for characters in other Square Enix works: Marlene Wallace and Mog, who are also cute, small characters.
Before names were released, Master Xehanort was referred to as OBM (Old Bald Man; in the vein of BHK/Blond-Haired Kid for Roxas). When his name was released, we got affectionate nicknames such as Baldinort, Baldsem, Geezernort, Oldynort, and Grampinort.
There's the Captain Obvious-styled Evil!Sora for Vanitas. The epilogue also gave birth to (as seen below) KH!Jesus for Sora.
As well as Jesusora.
A less common one for Vanitas: Soranitas.
Red Eye for the unnamed Heartless that serves as the boss of the Secret Episode.
Nomura considered showing Leon and Cloud's past in the game, but decided against it because the Radiant Garden would have been overloaded with characters. And because, in his words, this game is complicated enough as it is.
Also, a Jungle Book world was going to be featured, but was pulled out. As one of the cut rooms was Louie's throne room, it may have been axed because of Disney's Louie embargo (See the Screwed by the Lawyers page's entry on TaleSpin.)
Pleakley would've made an appearance in Deep Space.
The "Birth by Sleep: Volume Two" secret ending of Final Mix hints at a sequel for the PS Vita...that Nomura has now confirmed will not happen, as executive producers, for better or worse, thought there had been more than enough portable KH games, so the project was scrapped and focus was put solely on developing Dream Drop Distance and Kingdom Hearts III.