I think I have a good list of who could be in Organization XIII
1) Master Xehanort
2) Young Xehanort
3) Xemnas
4) Ansem, Seeker of Darkness
5) Xigbar/Braig
6) Saïx/Isa
7) Vanitas (it seems obvious that he would return as he was MX puppet in BBS)
8) No Heart (the lingering will of Xehanort fought in BBSFM, it appears to be alive and already is a Xehanort)
9) Terra-Xehanort (would be better than Apprentice Xehanort imo and already is a Xehanort)
10) Luxord (I put him here because out of all the non-returned Org. XIII members he seems to be the only one who could be part of the True Org. XIII. He also is the favorite Organization member of Nomura, or so I heard)
11) Riku-Ansem (I think he would work, but then they must find a way to fully suppress Riku)
12) Vexen Replica (Vexen turned out to be very valuable to the Organization but since he now is good Even again I think we could take the strongest replica Vexen created and make him into a Xehanort)
13) Oswald the Lucky Rabbit (my weird choice among all of them. I imagined Oswald being a keyblade warrior and took the Mark of Mastery Exam with Mickey but because of the darkness inside of him was rejected. So Oswald went mad and ran away and gave himself into darkness. And now he works with Xehanort just to bother Mickey. Likely to reform in the end).