It could be another powerful demon. Maybe a rival of Sesshomaru who has set up the barrier in the north to stop him from achieving his goals. This demon might be jealous of Sesshomaru's power or status.
The plot might center around Sesshomaru's journey to the north where he encounters this barrier. He could be on a quest for something, like a powerful artifact or to rescue someone. The barrier is in his way and he has to use his skills and powers to break through it. The 'fanfic' aspect allows for a lot of creative interpretations. It could include new characters or different relationships for Sesshomaru that are not in the original story.
Another possibility is that an old enemy of Chat Noir, perhaps someone he defeated in the past but who has now come back with a vengeance and a new power. This enemy could be using a form of dark magic to try and control Chat Noir, making him do things against his will. Chat Noir would then have to fight not only the physical attacks of this enemy but also the mental control they are trying to exert.
The antagonist could be a rival chef. This chef might be jealous of Chowder and Mung's success and try to sabotage their restaurant or steal their recipes.
A powerful psychic alien that has been lurking in the shadows could be the antagonist. This alien has been observing Ben for a while and decides to use hypnosis to make Ben do its bidding. The alien's motives could be related to some intergalactic power struggle, and by controlling Ben, it hopes to gain an advantage over its rivals.
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.
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.
There could be an ancient evil spirit that has a vendetta against griffins. When it senses that Cousland has befriended a griffin, it targets them both, trying to destroy their new - found bond and cause chaos in the world they inhabit.
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.
The Organization in the Claymore world could be the main antagonist. They already have a lot of secrets and do some morally ambiguous things. If Naruto shows up and starts changing the status quo, like teaching Claymore warriors to be more independent, the Organization might see him as a threat and try to eliminate him.
It could be a new alien species that Ben has never encountered before. This species could be territorial and see Ben as a threat to their planet where he's stranded. They constantly try to drive him away or capture him, which makes it difficult for Ben to find a way to leave.