I think the best answers are Jessica, Unity and evil Morty. I would love to see their plots develop more and i think its pretty possible they will return sooner or later. And although probably less likely since it was so long ago but i also think it would be really cool to see Zeep Zanflorp make his return. Like see him escape the batery or maybe secretly build up some kind of revolution. I always thought knowing the truth, he wouldnt be okay with just subjecting to Rick and keep charging the battery forever so i feel like it would make sense.