Well, i think that letter says something that has to do with bringing Sora and riku back and on to a brand new journey. Although, this time i think that Kairi might go along with them. And we might even get to see Kairi's world.......
Well.... I don't really think that there is any more secret ending things left since its been a long time since anyone found a secret ending. But if there really is one, then i hope for it to be revealed to us KH fans. xD