Did they deliberately avoid ever returning to the prior systems? Having side quests that send them back there would solve thus problem if it wasn't an intentional choice on their parts.

If it was, having the conSequences follow them would be another way to bring things to them. Have somebody come after them for revenge, for instance. Not for something they might expect, either. Those consequences they avoided dealing with fall on somebody else, and he or his loved ones come after the PCs for making that mess. That sort of thing.