There is perhaps no show in history that lends itself to as many fan theories as Rick and Morty. Most fans will be familiar with the theories that posit Rick and Morty are the same person, or that Evil Morty is Rick's original Morty. On a deeper level, there are some theorists who posit there are still new episodes of Rick and Morty Season 3, even after the "finale." They think there are secret episodes of Rick and Morty yet to be revealed. That last one is proffered by TheMistling, and if it takes you aback, remember that Season 3 started with a surprise episode, so who's to say "The Rickchurian Mortydate" was actually the last episode of Season 3?
Yes, there is a growing belief we have not yet seen every Rick and Morty Season 3 episode, and that theory is based on what we have seen so far this season, but maybe even more on what we didn't see. After all, Episode 10, "The Rickchurian Mortydate," didn't feel like much of a finale.
So, let's break down the theory that Season 3 of Rick and Morty is, in fact, not over. Be warned, though. There are some (rather tepid) spoilers within.
If you're initial reaction to this theory is "That's rick-diculous," don't forget that they had an unannounced season premiere on April 1st of 2017. Seriously, it makes way more sense from a marketing standpoint to add another episode at the end of a season, when people are still excited about the show. It would be much easier to get the word out in the latter scenario, and would provide incredible hype for Season 4.
Check out Dan Harmon's interview with Film Theory above, and make sure you catch the last minute.
In the last minute of the Film Theory interview, the interviewer asks showrunner Dan Harmon if they can discuss Evil Morty, and Harmon responds, "Just wait until the end of the season." Though he doesn't explicitly say Evil Morty will be in the season finale, it's heavily implied. Why else wait until after the season to discuss him?