I think we can pretty much cross Riku off the list at this point, he's done a good job dealing with the problem by himself.
So it really comes down to Terra but doesn't that just leave us back where we started? Separating Terra from Xehanort was always kind of a big question. Granted I'm skeptical that Xehanort's infection or whatever is really the normal process, I'd sooner think Riku was a bit of a special case because he lapped up all the darkness given to him like a thirsty dog to water. According to Riku "I took all the power you had and Sora still beat me" which seems to imply up until the final Riku vs Sora fight Riku was still willingly working with Ansem, or at the very least he was willingly letting Ansem use him like a puppet to beat Sora. But for someone like Terra who never accepted Xehanort, who fought against him at every turn, I get the sense he's probably not as deeply rooted into Terra as he was Riku and so splitting them should be a bit simpler of a task.
This is assuming that Terra isn't already split off from Xehanort. I mean he's had like 12 years and he did claim he had a plan to show Xehanort the door. Also isn't it a bit odd that he failed to rip Terra out the first time but later he was able to split himself into Ansem and Xemnas no problem? And of course there is the fact that Terra's entire being right now is up in the air seeing as he wasn't the one who appeared from Ansem and Xemnas's destruction.