It could be other students at Hogwarts who sense something strange about Snape and his invisible friend. They might try to expose Snape's secret out of jealousy or because they think it's some sort of dark magic.
Perhaps there is a powerful dark wizard from the past whose spirit is awakened by the presence of Snape's invisible friend. This dark wizard sees the invisible friend as a threat to his own plans for resurrection or domination and tries to destroy it, with Snape caught in the middle.
One idea could be that Snape's invisible friend turns out to be a manifestation of his repressed childhood self. It could help him face his past traumas and insecurities.
Well, it could be that in this fanfiction, Snape is written in a way that endangers Harry's friends. Harry is very protective of his friends, so if Snape is portrayed as harming them in some way, Harry would likely try to destroy whatever it is that's causing the threat. Also, it could be related to some prophecy or hidden knowledge in the fanfiction that makes Harry believe Snape has to be stopped.
If Izuku is an alien, the antagonists could be other aliens who want to use his unique powers for their own evil purposes. Maybe they see Izuku as a key to conquering Earth or other planets.
Most likely not. Popular fanfictions in the Harry Potter universe tend to focus on more relatable themes like relationships, adventures, or character backstories. This 'snot' - related concept is so far from the norm that it would probably only appeal to a very niche group, if any at all.
A possible plotline could be that Harry uses his invisibility to spy on Voldemort's followers and gather intelligence for the Order of the Phoenix. This would add an element of mystery and danger as he has to be extremely careful not to be detected.
Another antagonist could be a dark wizard or witch who wants to use the power associated with the Founders' heir for evil. They might target Hermione to either force her to join them or to extract the power from her. This antagonist would likely be very powerful and have a network of minions to carry out their plans.
One possible antagonist could be Orochimaru. He might be interested in Naruto's new Shadowman powers and try to steal them for his own evil purposes.
The Akatsuki could be antagonists. Even if they were defeated in the original story, in the fanfiction they might have a new plan and have kidnapped Naruto for some reason related to the tailed beasts. For example, they might want to use his Nine - Tails chakra to revive a powerful member or complete a dangerous jutsu.
One possible antagonist could be an alien bounty hunter who wants to take the Omnitrix from Gwen for his own purposes.
There could be a secret organization that has managed to turn the world against Naruto. This organization could be made up of rogue ninjas or people with a vendetta against the Uzumaki clan. They might spread false rumors or manipulate events to make Naruto look like a villain. Another possibility is that some of the old - school shinobi leaders who are afraid of change and Naruto represents a big change in the ninja world, so they oppose him.