Another potential antagonist could be a jealous rival within Slytherin. This character might be envious of the 'Prince of Slytherin's status and popularity and would stop at nothing to bring him down. This could lead to a lot of internal Slytherin house drama and power struggles in the fanfic.
Snape himself could also be considered as the 'Prince of Slytherin' in some fanfictions. Given his complex relationship with both Slytherin house and Harry, a fanfiction could explore a scenario where he is given this title due to his deep connection to Slytherin's heritage and his own role as a double agent, which in a way makes him a sort of prince - like figure within the Slytherin world.
There could also be other important characters like the Slytherin house - mates. Maybe a really cunning and ambitious friend who helps or hinders the main character's journey. And of course, there might be appearances from the more well - known characters from the original Harry Potter series, like Draco Malfoy, who could play a significant role in the fanfic, either as an adversary or an unexpected ally.
Maybe an older Slytherin student who has been influenced by the dark arts and doesn't want Harry to disrupt the status quo in Slytherin. This student could be envious of Harry's status as the WBWL and his growing influence in the house, and thus tries to undermine him at every turn.
The main antagonist could be a powerful necromancer who combines the dark arts of Skyrim with some elements of Voldemort - like magic. He could be raising an army of the undead and causing chaos across Skyrim, which Harry has to stop.
It could be a new dark wizard who has a vendetta against Harry. This wizard might use a spell to stop Harry from eating, thinking it will weaken him before a final showdown. He could be someone who was once a follower of Voldemort and now wants to take his place and prove himself by getting rid of Harry.
Maybe an ancient evil spirit that was once bound but is now trying to break free by using the summoner as a conduit. This spirit could be manipulating events from the shadows, causing chaos in the wizarding world to draw the summoner out.
Perhaps a corrupted Hogwarts professor. They could be secretly working for the remnants of Voldemort's followers. They discover that Eileen Prince had some knowledge or power that they want, and they see Harry Potter as a means to an end. This professor could use their position at Hogwarts to create all sorts of obstacles for Harry and try to manipulate him into leading them to Eileen's secrets.
Obviously, the 'Prince of Hell' himself (or itself). This character would be extremely powerful, with abilities that are far beyond what Harry has faced before. It could have control over dark magic in ways that Voldemort could only dream of.
Perhaps an ancient evil that Gryffindor once sealed away is starting to break free. This entity sees the descendant as a threat because they have the potential to reseal it. So, it tries to corrupt the descendant or stop them in any way possible.
It could be a dark wizard who also has some form of time - related power. Maybe he wants to use time to his advantage to take over the wizarding world. For instance, he could go back in time to eliminate the founders of Hogwarts so that he can rewrite the rules of magic for his own evil purposes.