There's a couple of psions. Yirix, the leader of the psion conclave, resents the guardians for defeating the psion sisters in season of dawn and worthy. There's also Amtec, the flayer responsible for throwing the almighty at the last city, and the last remaining sister of the flayers that we killed in dawn, whose status is currently unknown. I don't think either would make the list though since we haven't seen them in game.

The trouble I have is remembering which of them are still alive since they really only get spoken about in lore or in random strike throwaway dialogue. The psion rebellion in the season where crow saved Zavala seemed like it was setting something good up but I guess it doesn’t really have much to do with the larger narrative. Perhaps something for the episodes. Anyway. The more I think about it, this could be a great story for post final shape, an alliance of old enemies out for personal revenge. Be a good way to reskin the old ads to have it make sense that we are still fighting them too.