Rick Prime was never the series nemesis
It seems like a day doesn't go by in this subreddit when there isn't someone who expresses the view that the show is now aimless since Rick C-137's "greatest enemy" was killed unexpectedly in the middle of Season 7. Problem is, this isn't a show that's ever had one clear "nemesis," although fans could make a far more compelling argument for Evil Morty than Rick Prime - and EM isn't a nemesis either. Let's take a closer look.
Rick Prime wasn't even developed until the 6th season
To be fair, that's a bit of an understatement. The show worked on Seasons 5 and 6 simultaneously, meaning that they didn't begin to really flesh Prime out until both seasons were being written. Retroactively rewriting history is a real problem, because fans who either came to the series late or have the tendency to conflate events have forgotten that until the "Crybaby Backstory" in Rickmurai Jack, the events depicted in Rickshank Redemption had been conveyed as being entirely fictional.
The concept of "Rick Prime" only existed for 15 episodes
He only existed as a shadow over the series from Season 6's Solaricks until Season 7's Unmortricken. Evil Morty has been a "shadow" for far longer, yet everyone seems to be under the misapprehension that EM's story is "resolved."
Rick Prime served as a vehicle for a significant dangling plot point
The Omega Device was Rick Prime's greatest invention ... that, again, we never even heard of until the episode where Prime died. A considerable number of fans are operating under the misapprehension that Evil Morty's story is done, he got what he wanted and he's gone, but he has possession of the biggest example of Chekhov's Gun we've ever seen in storytelling.
So what does it all mean?
I sincerely don't think Evil Morty is a "nemesis" either. But he does have an immensely powerful weapon that has to be used at some point in the future. Again, this is the purpose of Chekhov's Gun - you don't introduce a gun in Act 1 without firing it by the end of Act 3. Doing anything else would be bad storytelling, and this show is built around strong storytelling.
It's entirely possible that we won't see Evil Morty inside the Curve again. I have strong hopes that when we encounter EM in Season 9, it'll be in one of two circumstances: either EM calls in a favor for having helped kill Prime and drags C-137 into a fairly innocuous adventure, or we see EM outside the Curve obtaining an "ordinary Rick" to have on hand for if he ever needs to use the Omega Device. (I've also speculated that EM could be a Morty who was abducted from outside the Curve - wouldn't it be interesting if the Rick he grabs is his Rick?)
But as far as a "show nemesis" goes, they've given us one, we've seen him at work frequently throughout the series, and he's a threat C-137 is going to have to deal with at some point if he truly loves his family and wants to keep them safe.
No, I'm not talking about Mr. PB. I'm talking about the "Infinite Rick" - the term Prime gave to represent the collective of Ricks who utilize portal travel and who now dwell within the Central Finite Curve.
If the Infinite Rick learns that the same Morty who destroyed the Citadel, killing possibly millions of Ricks and Morties in the process (though I suspect there weren't that many who lived there) came back to the Curve, killed the most notorious Rick of them all and then stole the friggin' Omega Device before leaving the Curve again, then it's hard to believe that this nemesis would simply let bygones be bygones. Instead I think we're ultimately going to see C-137 and EM working together against the Infinite Rick whenever that story gets told.
TL;DR
The true nemesis of the show is Rick himself - all Ricks everywhere in the Curve. Prime was only on our radar for 15 episodes, whereas there is still a considerable amount of story left to tell involving Evil Morty, who has a weapon that threatens all Ricks everywhere. For those of you reading this who see length and dashes and equate those things with AI, please feel free to run this through any AI detector you can find. AI doesn't write like me. I just wanted to get that out of the way up front, since lately with any lengthy post I write, there's someone who lacks the critical thinking skills necessary to eliminate the possibility that AI was used in the writing.