Putting spoiler tags on this one in chunks so people can spoil as far as they want to. Not that they're very big spoilers.
During the middle part of the credits,
there's a drop minigame that unlocks a secret message if you collect all of the glowing letters to spell out "Secret Message Unlocked" on the touch screen. The message says:
The past will be reawakened
as a new number
in never-before-seen detail
Prepare yourself for the awakening
of the next number
Which seems to indicate that
the next title will indeed be Kingdom Hearts III and the first Kingdom Hearts game on a current-gen console, likely the PS3 and maybe the Xbox 360 in North America.
Also, during the end part of the credits,
they do the usual thing of showing the characters doing things during the credits, except this time they're all accompanied by letters. If you put them all together, it spells "This leads to KINGDOM HEARTS". After that, the last two sets of characters to come on screen aren't accompanied by letters, but there's three characters in each shot. Specifically, DiZ, Roxas, and Xion in the first shot, and Terra, Aqua, and Ven in the second, so most of the people Sora is supposed to rescue.
I still need to get four more trophies to unlock the secret ending. Also, you apparently have to re-fight the final boss (
And there's a lot of them. As I recall, Riku has to fight Ansem, gets a reprieve at a save point, then immediately has to fight Young Xehanort and then the Armored Ventus Nightmare.) and then have the correct answers to three questions you get asked at the end.