Xion: I doubt it
Terra: I believe this is true
DS:....DS? Am I missing something?
Aqua: Does Kairi have a mom? She has a grandma, but nothing about her mom is known.
Ven: I believe the Ven and Roxas connection is linked to the unbirth's.
Maleficent: I doubt that
KH3: I hope not THAT late. I picture it being announced a year after BBS is launched in NA. My guess is a release date of 2013.
1st off I must emphasize that most of these are indeed just guesses & have no real supporting evidence.
Xion being a memory sure seems the simplest way of explaining things, how Kairi has 2 lookalikes & how she has her own Kingdom Key (it's just a memory of 1). Of course there is the matter of her existing before any of the other members got killed. Maybe she joined in the middle of CoM & the CO crew were called away for a quick improv meeting.
DS is Dark Soldier, the helmeted guy in BBS. The idea is that history between him & Ven is what begins the friendship of Roxas & Axel.
Maleficent strikes me as being the eventual final enemy of the whole series. She was originally meant the final boss of the 1st game umtil they came up with the idea of Ansem, so I think they may want to reprise her original role when they want to bring everything to an end. Speaking of whom, if Terra does indeed=Xehanort, then he'll likely be doing some redemption thing in KH3 & can't really be the final boss of it. Besides I just want to see a Disney villain actually
be the ultimate threat for once.
The team behind KH1 & 2 are developing FFvs.XIII, which won't come out until after FFXIII, which probably won't come out itself until late 2009, so development on KH3 won't even begin until 2010 or 2011 at that rate. & since it's expected to be bigger & prettier than KH2, it'll take longer to develop.