It could be the classic villains from the original series like All For One. He might be interested in baby Deku's potential quirk. Another could be Shigaraki, who might see baby Deku as either a threat or an opportunity to disrupt the hero world in a new way.
One interesting concept could be the idea of baby Deku having unique interactions with villains. Maybe the villains are unexpectedly protective of him, which is a contrast to their usual evil nature. It could also explore how baby Deku's innocence affects the villains' plans or makes them question their actions.
There could be a new, unknown villain. Maybe a dark force from a far - off land that senses the power of Mario and Peach's baby and wants to use it for its own dark deeds. This villain could bring in new elements to the fanfiction, like different types of magic or strange creatures that the Mario characters have to face to protect their baby.
There's a group of villains in the 'Villains Rising Light Novel'. One of the main ones is a woman named Lilith. She is cunning and uses her intelligence to outwit her enemies. Another is a hulking brute named Brutus who uses his physical strength to cause chaos. They all have different motives for their villainy which makes for an interesting dynamic.
I'm not sure specifically as I haven't read the exact novel. But usually, the main villains are those who oppose the protagonist the most. They are often the ones causing the most chaos and problems in the story world.
Salem is also a very popular villain in RWBY fanfiction. Given her status as the main overarching villain in the series, fanfic writers like to expand on her backstory, her motives, and her long - term plans. There could be fanfictions that focus on her interactions with other villains in a more detailed way.
It could be a variety of villains. Maybe some of the traditional Naruto villains like Orochimaru or Pain could be in the story and Naruto kills them. Or perhaps in this fanfiction crossover, there are new villains created that are a combination of Naruto - style threats and Justice League - type menaces. It's hard to say exactly without reading the fanfiction, but it would be interesting if it included some well - known Naruto baddies being taken out by Naruto in a new setting with the Justice League around.
The Volturi could be the villains again. Since they were so concerned about Renesmee, they might be even more worried about a new half - vampire - half - human baby. They could see it as a threat to their power and their rules about vampires and hybrids.
One likely villain could be Loki. He's always up to no good and loves to play mind games, so a card game would be right up his alley. He might be the one who lures Tony into the game.
One could be Ozai from Aang's world. He might be even more powerful in this fanfiction and Naruto has to step in to help Aang deal with him. Naruto's unique skills and abilities would be crucial in countering Ozai's firebending.
Another one is Goro. He's a petty thief in Naruto fanfiction who has a special skill of being able to blend into the shadows. He steals from local merchants in Konoha. Goro may not seem like a big threat at first, but his actions can sometimes lead to bigger problems, like when he steals important items that are needed for a mission.