It could be a powerful chef who has been corrupted by the desire for ultimate power in the cooking world. This chef might see Naruto and Alice as threats to his dominance, especially if they start to gain popularity and win Shokugeki battles. He could use underhanded tactics like stealing their recipes or sabotaging their ingredients.
There could be a rival from Alice's past, someone who was jealous of her talent and has now joined forces with a dark power. This antagonist would be constantly trying to undermine Alice and Naruto. They might create false rumors about their cooking techniques being unethical or try to disrupt their cooking during important Shokugeki battles.
Well, perhaps in this fanfic, Naruto is sent to a parallel world where he meets Alice from the Shokugeki world. He has no idea about cooking at first, but he's eager to learn because he wants to fit in. Alice takes him under her wing. They then enter a special Shokugeki tournament where the rules are a bit different. Naruto's ability to think on his feet and adapt quickly helps them come up with unique cooking combinations. For example, he might use his chakra to enhance the flavors in a way that no one has ever seen before in the Shokugeki world.
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.
Another option could be a corrupted version of Asriel. Maybe Alice's arrival somehow stirs up the darker parts of his past and he becomes a force against her, believing she is a threat to the delicate balance in the Underground.
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.
It could be Darth Sidious. Since he is always looking for power and control in the Star Wars universe, he might see Naruto as a threat to his plans when Naruto arrives in Padme's world. He would try to eliminate Naruto and use Padme as a pawn in his political games.
It could be Orochimaru. He might be interested in Baby Beel's powers and try to capture him for his own evil experiments. Orochimaru would see Beel as a unique specimen and would stop at nothing to get his hands on him. Naruto would then have to use all his skills and the bond he has with Beel to keep him safe from Orochimaru.
It could be Ra's al Ghul. He's always interested in things that can give him an edge in his quest for immortality. If he discovers the power of chakra in the Naruto world, he might try to take over both worlds. His skills as a master assassin and his vast resources would pose a huge threat to Naruto and Batman, who would have to put aside their differences to defeat him.
A powerful dark ninja who has made a pact with some dark force from another dimension. He sees Gabriel as an intruder and Naruto as a threat to his plans of taking over the ninja world. He would use his dark jutsu and the power from the other dimension to oppose Naruto and Gabriel, and would stop at nothing to achieve his goals.
It could be Orochimaru from Naruto's world who has found a way to cross into the DC universe. He teams up with some of Batman's villains like the Joker. Orochimaru wants to use the chaos in Gotham to conduct his dark experiments and the Joker just wants to cause mayhem. Batgirl and Naruto would have to deal with Orochimaru's snake - like attacks and the Joker's unpredictable madness.
Most likely Danzo would be seen as the main antagonist. His actions throughout the Naruto series were often self - serving and harmful to others, like his role in the Uchiha massacre. So when Naruto talks to him, he would be the one Naruto has to oppose or at least try to change.