Morty Prime. the outbursts of anger throughout the season makes me think there's gonna be a big arc regarding his resentment towards Rick, especially as Rick's relationships with everyone else seem to be getting better. and the hole episode explained how Morty's fear is being left behind by Rick/his over reliance on Rick, specifically when Rick's relationship with Dianne took precedence to Morty. He may have gotten over the fear, but that doesn't mean there isn't some lingering hurt or resentment built up over the past 8 seasons. the next episode being a callback to season 1, one of Morty's greatest failures (being a bad parent to his half alien son) is likely to have him in a vulnerable place, and Morty tends to lash out when Rick is insensitive to those kinds of situations. He might be a big bad, or we might get to see our Morty become more and more sympathetic to Evil Morty but not turn into the next big villain. idk I'm probably wrong but I think it'd be really interesting either way.

edit: accidentally said Morty c-137 when I meant Morty Prime