I for some reason get the implication that Nomura was referring to mostly antagonists like Xemnas, Ansem, and Master Xehanort when he was referring to Dark Seeker saga characters who won’t return. The former two in particular seem really dead this time and I don’t see MX returning outside of YMX. That’s just the end of the road for them - they were defeated, they finally realized how much their lives sucked, but it was too late for them. Riku Replica probably won’t return either. Every other character I could see returning, but it’s likely some will indeed take a big backseat for a while and would only return for the final battle against MOM or something.