I consider Repliku a likely guess. The way Riku sounds about the "other me" is rather genuine and caring, so I doubt it would be Ansem. Planning ahead if he was to fall to darkness seems rather unlikely to me because if he was to fall into the darkness so late in the series, it would be kinda dumb. A lot of his development focuses around him NOT falling into darkness even though he always had been really close to it, so it would be stupid to make him give in at the peak of the story. Plus, Mickey is with him and should be doing something for the first time in years, so yeah. I always wondered he never produced a heartless / nobody anyway, but it would be a terrible timing to reveal it now if he had because it would imply it had never been discovered until this day which would mean a ton of retconning incoming.
Repliku had been a rather tragic figure in Re:CoM and even though Riku wasn't exactly fond of finding a clone of himself, I guess he would take pity. Maybe Riku even knows something we don't, maybe Repliku is a SoD and Riku is preparing for that case. For my part, I'd love for it to be Repliku, that guy deserves more love.