So for the currently revealed darknesses we got:
Master Xehanort
Young Xehanort
Ansem, Seeker of Darkness
Xemnas
Xigbar
Saix
I feel that these characters could be potential darknesses:
Riku-Replica
KH1 Riku
Terra-Xehanort
Marluxia
Larxene
Vanitas
Ventus
The Rikus and Vanitas will be time travel shenanigans.
Marluxia and Larxene could be consequences for trying to overthrow the organization.
Aqua probably takes Ventus out of his sleeping chamber when she's rescued then Xehanort snatches him up or something.
It really is hard to predict who the darknesses will be though. The only one I'm really sure of out of what I wrote is Marluxia because of his new Nobody spawn in one of the KH3 trailers but it is most likely nothing.
Hell make Marluxia return in any context.
Terranort's most likely going to be one of them. MX has obviously tried (and occasionally succeeded) to steal light-aligned keyblade wielders over to his side in order to disrupt the seven lights so he can go for the easy-mode attack-the-princesses solution, though, so potentially we'll see Sora do the same thing by rescuing Terra before the endgame. The time travel shit technically kinda allows Terra and Terranort to both be in the mix at the same time, I suppose.
Ventus is pretty out of the question as far as darkness is concerned (his damaged-ass heart is pure light). I think Vanitas is an easy and obvious candidate, though.
MX, YX, Terranort, Ansem SoD, Xemnas, Xigbar, Saix, and Vanitas are the only characters we've seen with the trademark yellow eyes (Riku and the Riku Replica never did, and neither did any other Organization XIII members), though, so... who knows who else it'll be. I wouldn't be too surprised to see KH 2.8 shed some light on this (especially with the Forerunners shit).