Warning: The following contains SPOILERS for Rick & Morty season 5, episode 3, "A Rickconvenient Mort."
Rick and Morty season 5 episode "A Rickconvenient Mort" teased a secret history of superheroes operating on its version of Earth. This is notable as most episodes have suggested that its world was largely like the real world, up until whatever weirdness Rick was dealing with that week ran out of control. Yet "A Rickconvenient Mort" suggested that superheroes have been a common thing since at least the 1990s and made reference to there being at least four phases of superheroes.
The main story of "A Rickconvenient Mort" centered around Morty developing a romantic relationship with Planetina; a gender-flipped parody of Captain Planet. A protector of the environment when she wasn't doing personal appearances or autograph fairs, Planetina was presented as a typical superheroine who fought supervillains like the pollution-loving Diesel Weasel. Various news reports throughout the episode reported on Planetina's activities and didn't act as if there were anything unusual about a woman who channeled the power of the elements.
Granting that a superhero born of the combined powers of four magic rings isn't that strange of a concept for Rick and Morty, it doesn't fit within the show's general theme either, where the insanity of Rick Sanchez's adventures encroaches on the mundane reality of the Smith family. While the universe of the show is full of wonders and weirdness, it usually skips over Earth until Rick becomes involved. This seems to have changed in Rick and Morty season 5, which opened with an episode revealing the existence of Rick's nemesis, Mr. Nimbus, a parody of Marvel Comics' Namor the Sub-Mariner.
Rick and Morty has parodied superheroes before, most notably in season 3 episode "Vindicators 3: The Return of Worldender," which introduced the Vindicators team that is getting their own spinoff series. However, the Vindicators were an interstellar superhero team, comparable to the Guardians of the Galaxy, who didn't seem to have much to do with Earth despite having some apparently human members. This makes it all the more jarring the media and civilians in "A Rickconvenient Mort" didn't consider Planetina to be something unusual or that Summer referred to her being "a Phase four superhero."
While this terminology evoked Phase Four of the Marvel Cinematic Universe, it was never explained just what it meant within the context of Rick and Morty. However, the fact such terminology exists to define superheroes, regardless of what phases refer to in this case, suggests many such superheroes and supervillains are running around. Perhaps more characters like Mr. Nimbus and Planetina will continue to pop up in Rick and Morty season 5. At the very least, there will likely be some sort of backdoor pilot to set up the Vindicators spinoff.