Honestly, I can see something like this happening.....
1. MX
2. Xemnas
3. Ansem
4. Xigbar
5. YX
6. Saix
7. Repliku
8. Vanitas
9. MX Nobody
10. Marluxia
11. Luxord
12. Terra
Not in that order of course, but something along those lines as a possibility.
I shall supply my reasoning and not just leave ya hanging...
Repliku
Now this could just be any form of Riku's Dark Side, but whatever form it may take, I believe it could be one of the Seekers of Darkness. I'd like to believe it is Repliku.....*stares down at sig*
Vanitas
The same reason others have probably said. He's Ven's Dark Side, and meeting between Sora and Vanitas would be something to see.
MX Nobody
It was said that when MX took his heart out of his body and put it into Terra, that something was left behind. MX had created something, but it wasn't exactly a run-of-the-mill Nobody.
Marluxia
Even though he turned traitor in Castle Oblivion, if you remember in Days when he was still around, he primarily talked about the Keyblade and how it was a marvelous weapon. He was fascinated with it when working with Roxas. (I'll replay and see what he says exactly).
If Marluxia wants a Keyblade, then joining the SoD's would be a way. His reason for being one would be similar to that of Xigbar's.
Luxord
Why Luxord? One reason, is that he's Nomura's favorite. But that's not the only reason.
Luxord is all about gambling and bets, when the stakes are high that's when things get exciting.
Just think, being a SoD would mean the ultimate gambit. He's throwing his individuality and self on the line for a goal.
Now for Marluxia and Luxord, there is the argument "they don't seem to look like Xehanort, like Saix or Xigbar do".
It was also said that out of the remaining Seekers, Saix and Xigbar were the two farthest down the line in the transformation. They had progressed the most.
Luxord and Marluxia are two of the latest members of the original Organization, so they wouldn't have had Xehanorts heart within them for that long.
When I think of something else, I'll throw it out there too.
Why do I suddenly imagine Xehanort lying in a cradle with a baby's rattle uttering his very first words. *ggg*
"Da-da-da...X-blade." lol.
When I read this part, I imagined a baby saying it. Then I got to
X-Blade, and thought of Nimoy's voice coming out of a little baby.