Obviously, Cupid could be the main antagonist. If he's being a nuisance and interfering in the characters' lives against their will, he fits the role of the bad guy in this scenario.
Another possibility is that Buffy's own inner demons regarding love could be the antagonist in a sense. Cupid's actions just bring these issues to the surface. She has to battle her own insecurities and fears about love and relationships, which are made worse by Cupid's meddling. So, it's not a traditional antagonist but still something that stands in her way.
Perhaps in this fanfic, Cupid has been causing chaos in Sunnydale. Buffy has always been about fighting evil, and she starts to see Cupid as another form of annoyance. She might start to investigate why Cupid is being so out of line. She could find out that there's some dark force influencing Cupid, and she has to deal with that while also dealing with her own feelings about love and relationships. It could be a really interesting exploration of Buffy's character and her views on love, which are often complex given her life as a slayer.
The antagonist could be a jealous colleague. This person sees the budding relationship between Chuck and Bryce Larkin and is envious. They start to spread rumors and create problems for Chuck and Bryce, trying to break them apart. Maybe this colleague wants Bryce for themselves or just doesn't like the idea of Chuck and Bryce being together.
The First Evil could also be a great antagonist. It could try to corrupt the baby, seeing it as a new vessel to spread chaos. Since the baby would be part of Buffy and Faith, it would be a prime target for the First Evil's schemes.
Perhaps a group of anti - technology extremists could be the main antagonists in an Ultron fanfic. They see Ultron as the symbol of all that is wrong with technology and are determined to take him down, no matter the cost. They might use their own advanced, but more rudimentary, technology to fight against Ultron. This would create an interesting contrast between Ultron's high - tech abilities and their more basic, yet effective, methods.
It could be Naruto and Hinata. Since they have a relationship in the original series, a fanfic about their first kiss would be quite popular.
Well, first off, Psyche's great beauty made Venus jealous, so Venus sent Cupid. But Cupid fell for Psyche. They lived in a hidden palace, but Psyche was curious about Cupid's face due to her sisters' influence. When she saw him, he left. After that, Psyche had to go through many tough tasks. She had to sort out different grains that were all mixed together. She also had to get the golden fleece from some really dangerous rams. These were the main events that eventually led to their reunion.
It could be a powerful and jealous student. This student might be envious of Natsu's popularity and strength. So, they start causing trouble in the academy, trying to undermine Natsu at every turn. They might even form a group of followers who are also jealous of the other popular students in the academy.
It could be a powerful vampire coven. They might see Buffy and the wet works team as a threat to their plans of taking over the city or a particular area. The vampires could use their powers of seduction and mind control to try and turn the wet works team against Buffy or vice versa.
A rival medicine cat from another Clan might be the main antagonist. They could see Alderheart as a threat to their own status or the well - being of their Clan. So they might use dark magic or some underhanded tactics to bring about his death.
Vilgax could be a great antagonist. He could hack into the gaming systems to try and defeat Ben in a virtual world where the rules are different. He might use his advanced technology to create digital versions of his minions to attack Ben.