Maybe it's a new character created specifically for this fanfiction. A powerful alien scientist who sees Morty and young Rick as a threat to his plans for universal domination. He would constantly be trying to stop their adventures and capture them.
There's a possibility that an older Morty from a different timeline becomes the antagonist. He's bitter about his own life and doesn't want this Morty to have a good time with young Rick, so he tries to disrupt their relationship and the positive experiences they're having.
In this fanfiction, Morty might be really shocked at first to meet a young Rick. He could be confused about how this young version of Rick behaves compared to the Rick he knows. Maybe they'll go on some wacky adventures together that are different from the ones Morty has with the regular Rick.
The League of Shadows could be a great antagonist. They might see Young Justice as a threat to their plans in the world and Batgirl as an obstacle in Gotham. They could try to manipulate situations to make Young Justice and Batgirl turn against each other while they carry out their own devious plans, like releasing a dangerous virus in a major city.
It could be a new group of survivors who are extremely territorial. They see Rick's group and Clementine as threats to their resources. This group might be led by a power - hungry man who is willing to do anything to protect what he thinks is his, including attacking Rick's group and trying to capture Clementine.
Well, in the world of fanfiction, people can take characters in all sorts of directions. But in the context of 'Rick and Morty', the idea of Rick abusing Morty in the fanfiction is a deviation from the show. The show focuses on their interdependent relationship, with Rick being the brilliant but flawed mentor and Morty being the somewhat naive sidekick. Fanfiction writers might create such scenarios to explore the darker side of their relationship, but it's not a part of the main narrative.
It could be some alien she met during one of Rick's adventures. Maybe she got involved with a being from a different dimension who had a human - like form and they had a brief encounter that led to the pregnancy.
It could be a powerful warlock who has found a way to manipulate time and space. He wants to stop Paige and Prue from joining forces because he fears their combined power. So he creates various obstacles and illusions to keep them apart.
Maybe a powerful rogue ninja who has discovered a way to steal the powers of divine entities. He sets his sights on Amaterasu and Naruto because he knows that if he can combine their powers, he will be unstoppable. He creates various traps and illusions to separate them and weaken them so that he can carry out his evil plan.
A powerful new pirate group could be the main antagonist. They might be after the same treasure or territory as Luffy and Shanks. They could have some unique and dangerous abilities, like the ability to control sea monsters. Their leader could be a former crewmate of a well - known and feared pirate, who has his own vendetta against both Luffy and Shanks for some past reasons.
There could be an ancient force that is awakened by the meeting of Harry and Hela. This force wants to destroy both of them because their combined powers could potentially stop it from achieving its goal of spreading chaos and destruction throughout all the realms. It would use all kinds of devious means to try and separate them and defeat them one by one.
Maybe a powerful warlord from Negan's world who somehow follows him into Artemis' realm. This warlord, with his own thirst for power and destruction, would oppose both Negan (if he starts to side with Artemis) and Artemis herself, wanting to take over the new world he has found.