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.
Well, they could have a plot where after getting married, they start a small business together. They have different skills - Brady might be good at marketing and Mikayla at product development. At first, they struggle to get the business off the ground. There are issues with suppliers, competition, and financial problems. But with their love and determination, they gradually build a successful business and become an inspiration in their community.
In this fanfiction, Brady and Mikayla's marriage could be a way to unite two different groups on the island. Their wedding day might start with some comical mishaps, like Brady's suit getting dirty right before the ceremony. But in the end, they have a beautiful sunset wedding on the beach, with the sound of the waves in the background as they say their 'I do's'.
A great plot point could be that an old enemy tries to disrupt their wedding. They have to fight to keep their special day intact. Another could be that they discover a hidden family secret during their wedding preparations that changes how they see their relationship.
In the fanfiction, the kiss could be a turning point. It could signify a change in their relationship, perhaps from friends to something more. For example, after the kiss, they might have to face the reactions of their friends and family, which could add a whole new layer of drama to the story.
I'm not sure who the specific best authors are, but you could start by looking at the most popular fanfic platforms. Check out the ones with the highest ratings and most reviews. They are likely written by good authors.
It could be that Brady and Mikayla are in a school setting. They've been study partners for a while and slowly developed feelings for each other. One day, after a particularly tough exam, they are sitting outside, and Brady just leans in and kisses Mikayla out of the blue.
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.
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.
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.