It could be a new Sith Lord. This Sith Lord could be someone who has been in hiding for a long time, gathering power and knowledge. They see Ashoka and Yoda's reunion as a threat to their plans of taking over the galaxy. So, they start to target them and try to disrupt their new - found bond.
Maybe an ancient dark side entity. It was sealed away long ago, but due to some recent events in the galaxy, it has been awakened. It doesn't want the light side represented by Ashoka and Yoda to gain strength again, so it sets out to stop them. It could have the ability to possess other beings, making it a very difficult enemy to fight.
A powerful warlord who has amassed a large army. He wants to rule the galaxy by force and sees the Jedi, even the few remaining like Ashoka and Yoda, as obstacles. He is not Force - sensitive but has a lot of military might. He tries to use his army to capture Ashoka and Yoda and use them as leverage against any other Jedi or Force - users who might oppose him.
Well, perhaps the plot could involve Ashoka seeking Yoda's wisdom after a long and arduous journey. She might be facing a new and powerful enemy that she doesn't fully understand. Yoda, with his centuries of knowledge, could offer her insights on the Force and how to combat this threat. Maybe he would teach her a new way to use the Force that she hadn't considered before, leading to a final showdown where Ashoka is able to defeat the enemy using Yoda's teachings.
It could be Voldemort who somehow manages to cross over into the Star Wars universe. He might see the power of the Force as something to be harnessed for his own evil purposes. His dark magic would be a great contrast to the light - side teachings of Yoda, and Harry and Yoda would have to team up to stop him.
It could be a powerful pirate who has a grudge against Ace and Sabo's crew. This pirate might have some past history with them, like they were defeated by Ace and Sabo's crew in the past and now wants revenge.
A Sith lord who has been secretly observing Ashoka could be the main antagonist. This Sith lord might see Ashoka's potential and want to turn her to the dark side to use her power for their own evil plans.
Perhaps an evil organization that wants to harness the power of both Naruto and Nero. They might see Naruto's chakra and Nero's Devil Bringer as the keys to ultimate power. This organization could be led by a powerful sorcerer who can manipulate dark magic and has minions from both worlds at his disposal.
Maybe a new character created specifically for this fanfic. A powerful sorcerer who wants to use Naruto's chakra and Slade's combat expertise for his own evil purposes. He could be manipulating events from the shadows, pitting Naruto and Slade against each other at first until they realize they are being played.
Perhaps Mister Sinister. He has always been interested in mutants like Wolverine and could see Deadpool as a useful pawn or a nuisance in his grand schemes. He might try to manipulate the situation to his advantage, causing Deadpool and Wolverine to join forces against him.
It could be a powerful, corrupted Pokemon trainer who wants to capture Cosmog and use its unique powers for his own evil purposes. He might see Cosmog as a key to unlocking ultimate power and doesn't care about the harm it could cause to Cosmog or the Pokemon world. He could also be in league with some dark - type Pokemon that can create obstacles for Marshadow and Cosmog.
It could be Thanos. Since he is a powerful and menacing force in Loki's universe. He might see Harry as an unexpected variable and want to eliminate him. Also, his goal of collecting the Infinity Stones could put him at odds with both Harry and Loki as they might try to stop him to protect their respective worlds.
Mr. Burns could potentially be the antagonist. He might see the combination of Spongebob and Homer as a threat to his power in Springfield. He could try to separate them and cause trouble for them. Maybe he wants to take over Bikini Bottom as well, seeing it as a new source of resources or power. He starts by spreading rumors about Spongebob and Homer in both their hometowns to turn the locals against them.