And Young Xehanort.
There obviously has been a lot of predictions regarding the other vessels, but I've got a few names to fill in five of the seven blanks.
1. Post-DI, pre-HB-keyhole KH1 Riku. This is the version of Riku with the weakest heart. He was jealous of Sora, the Keyblade, and his newfound friends, so much to the point that Maleficent was able to sweet-talk him into joining the dark side. If she could, Xehanort could have, too, and who knows what he had been doing off-screen; for all we know, Xehanort might have pulled him out of KH1 into DDD (and KH3) to witness all those events.
2. Dilan / Xaldin. During his time as a Nobody, it was evident that he absolutely loathed humans and saw their emotions as weakness. He seemed to have actually "enjoyed" having no heart. Also, in DDD, it was revealed that his condition upon awakening was unstable; while this could mean anything or nothing at all, can we really say for sure that he's unstable not because of Xehanort's heartling doing its job?
3. Even / Vexen. He's here because of his being unstable, just like Dilan. In addition, his intelligence might be needed by Xehanort, just like his Replica Program was an invaluable asset to Xemnas. Please note that the mention of Replica Program was there solely for analogy purposes, and not indicative of it making a return in KH3.
4. Luxord. In my opinion, he was one of the members of the former Organization who were loyal to Xemnas. He was the third last to be defeated, even after Xigbar. Despite being the tenth person, he certainly gave off an impression that his standing was high, unlike Demyx.
5. Vanitas. Master Xehanort created this being singlehandedly. To him, Vanitas was just a tool to forge the X-blade, just like Ventus. I think it's not entirely impossible that he would reprise his role on a bigger scale. In making this assumption, I'm actually ignoring the heights of the new members that appeared in DDD to be similar.
As for the remaining two, I'm still not sure who they will be. However, I'd bet that Sora's replacement, whoever that may be, would hold the most surprise when he or she is revealed in KH3.
Gosh, I needed to look for this in FI. It's all thanks to that troll member back then that this my wall of text became moot. <_<