The main antagonist could be a jealous acquaintance of Cosimo. This person has always been envious of Cosimo's status and charm. When he sees Cosimo falling for Contessina, he decides to sabotage their relationship. He starts by spreading malicious lies about Contessina, making Cosimo question her true nature.
One aspect that makes a Cosimo and Contessina fanfic interesting is the potential for character development. Cosimo could start as a young, ambitious man, and through his relationship with Contessina, he becomes more empathetic and wise. Contessina, on the other hand, might grow from a sheltered noblewoman to a strong and influential figure, all thanks to their bond.
Maybe in the fanfic, Contessina is a talented artist and Cosimo is a patron of the arts. Cosimo discovers Contessina's work and is immediately captivated. He commissions her for a large project at his family's estate. During the process, they fall in love. But there are rumors and scandals that swirl around their relationship as it goes against the social norms of their time. They have to fight against the gossip and prove that their love is true.
A Cosimo and Contessina fanfic could be set in the vibrant backdrop of the Renaissance. It might explore their relationship as they navigate the political and social intrigues of the time. Maybe Cosimo is a powerful merchant and Contessina is from a noble family, and their love is a secret that they must protect.
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.
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.
It could be a rogue ninja who was once a student of Orochimaru and has now developed his own dangerous jutsu. He wants to take over the ninja world and believes the old ways are holding everyone back.
Orochimaru could be a great antagonist. He might want to use Shenron's power for his own dark experiments. He could try to steal the dragon balls and manipulate the wishes to gain immortality or create an army of powerful, immortal ninja.