A mad scientist who is obsessed with creating a spotted animal army could be the main bad guy. He sees the 101 Dalmatians as perfect specimens for his experiments. Thunderbolt discovers his plans and has to stop him, all while protecting the Dalmatians from being kidnapped by the scientist's henchmen. This antagonist could have a high - tech lair and all sorts of crazy inventions to make things difficult for Thunderbolt and the Dalmatians.
A fanfic could focus on Thunderbolt befriending one of the 101 Dalmatians in particular. Let's say he meets a Dalmatian pup who has big dreams of being as famous as him. Thunderbolt then takes this pup under his wing and shows him the ropes of the showbiz world within the context of the 101 Dalmatians' universe.
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.
One option could be an alien scientist who is obsessed with the power of the Nemetrix. He wants to study it and use it to create his own super - weapons. He might be from a race that has been at odds with the creators of the Nemetrix for centuries.
It could be someone who opposes Naruto's newfound admiration for Ganondorf. Maybe a traditionalist ninja who thinks Naruto is straying from the true ninja path by getting influenced by Ganondorf.