Perhaps a new detective rival could emerge as an antagonist. This person might be jealous of Shinichi's success and his happy married life with Ran. So, this rival tries to undermine Shinichi's reputation in the detective world by framing him for crimes he didn't commit, which would put a strain on Shinichi and Ran's relationship as they deal with this new threat.
It could be an old enemy from Shinichi's past cases who has a vendetta against him. Now that Shinichi is married to Ran, this enemy decides to target their new family. They might try to create misunderstandings between Shinichi and Ran, like sending false information or causing problems in their personal and professional lives to break them apart.
The Black Organization could still be a major antagonist. Even though Shinichi and Ran are married, the Organization might try to target Shinichi because of his knowledge of their secrets. They could try to use Ran as a way to get to Shinichi, which would create a lot of tension in the story.
One possible plot could be their life after marriage. Shinichi might still be involved in some detective cases, and Ran would be his strong support at home. She could also get involved in some cases with him, adding a bit of adventure to their married life.
The priesthood of an ancient Egyptian cult that has been forgotten but still has some lingering power could be antagonists. They believe that the union of Sadie and Anubis is against the old ways and will bring bad luck or some sort of cataclysm. They might use dark magic to try and separate Sadie and Anubis. Also, there could be a jealous god or goddess, perhaps Isis, who is used to being in a position of power and influence. She might see Sadie as a threat to her status, especially if Sadie's relationship with Anubis gives her some sort of special powers or influence in the divine realm.
It could be an ex - partner of either Yuno or Yuki. For example, if Yuno had a really possessive ex, that person might try to break up their marriage out of jealousy. They could spread false rumors or create misunderstandings between Yuno and Yuki.
Another option could be a business rival if they are involved in a business in the fanfiction. This rival might try to steal their ideas, undermine their reputation in the market, or even try to break up their marriage in order to gain an advantage over them in the business world.
Maybe a powerful wizard or sorceress who has a grudge against Jaune Arc or the fox's kind. This antagonist could try to use magic to break up their marriage or cause trouble between them. For example, they could cast spells to make the fox lose her memory of Jaune Arc or create illusions to make Jaune Arc distrust the fox.
The main antagonists could be some old enemies of Percy or Reyna who don't want to see them happy. For example, a vengeful monster from Percy's past might try to disrupt their relationship. Maybe it's a Titan who was once defeated by Percy and wants to ruin his newfound happiness with Reyna.
Some ancient and powerful monsters could be antagonists. Since Percy is now connected to the Fates, any disruption in the natural order might attract these monsters. They could see the situation as a chance to gain power or revenge. For instance, a powerful Titan who was once imprisoned might see Percy's new status as a way to get back at the gods by attacking him.
Some pure - blood supremacist families who don't approve of their marriage could be antagonists. They might try to cause trouble for Harry and Luna, maybe by spreading false rumors or trying to undermine their work in the wizarding world.
One common antagonist could be an old enemy of Artemis who doesn't approve of her marrying Percy. This enemy might be a powerful Titan who sees their union as a threat to his plans for revenge against the gods.
The Sith Order could be one of the main antagonists. They might see Vader's marriage to Padme as a weakness or a deviation from the Sith path. So they could try to undermine their relationship or even harm Padme to bring Vader back to their full control.