13 Seekers of Darkness:

1. Master Xehanort

2. Xemnas

3. Ansem

4. Xigbar

5. Terra-Xehanort

6. Riku Replica

7. Saix

8. Demyx

9. Luxord

10. Marluxia

11. Larxene

12. Young Xehanort

13. Vanitas

 

Imagine 4,6,7,8,9,10 and 11 with white or silver hair and the same arrange to suit Xehanort's style of wearing his hair and yellow eyes.

 

10,11 I know traitors they can't be I just can't think of more suiting vessels than these two.

 

Even Riku Replica gets to join with Vanitas revival no character is left behind (Since the characters of the light aligment are going to be restored why not all the characters of the dark alignment?)

 

Possible Guardians of Light:

1. Sora

2. Riku

3. Kairi

4. King Mickey

5. Ventus

6. Lea

7. Aqua

8. Terra

9. Roxas

10. Xion

11. Namine

12. Master Eraqus

13. Donald

14. Goofy

15. Yen Sid

Edited June 29, 201312 yr by geocenter

I highly doubt Marluxia, Larxene or Demyx will be included, like you said they'd make nice vessels (people'd like to see them and fight them) but the first 2 were traitors and the former was weak.  Luxord is the only currently unseen Organization member who fits the criteria of not being a traitor or weak.

 

I pretty much agree on all of them except those three, I'll include my possible substitutes/people I'd want to see as darknesses below:

 

1. Master Xehanort
2. Xemnas
3. Ansem
4. Xigbar
5. Terra-Xehanort
6. Riku Replica
7. Saix
8. Demyx Zack (Hasn't been seen since KHBBS, wanted to be like Sephy, could surprise some)
9. Luxord
10. Marluxia Sephiroth (An obvious darkness, in terms of power and darkness he'd be a prime candidate, but whether he's likely can be disputed)
11. Larxene Oswald (Mickey's brother, King of the Wasteland)
12. Young Xehanort
13. Vanitas