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.
Well, a plot could be centered around Eileen Prince's family secrets. Suppose there were some hidden prophecies or family legacies that she was aware of. In the fanfiction, Harry could accidentally stumble upon these secrets while researching Snape's past. This could lead to a whole new adventure where Harry has to deal with both the revelations about Eileen and the implications for the wizarding world at large.
A great fanfiction idea could be centered around Eileen Prince's family. It could delve into her early life, her relationship with her parents, and how that influenced her decision to marry into the Snape family. This could also explore how her family's values were passed down through generations in the wizarding world.
Perhaps a long - lost relative of the Potter family who feels they were unjustly excluded from inheriting the manor. This relative could be jealous of Harry's status and the wealth associated with the manor. They might use dark magic to try and claim what they think is rightfully theirs, leading to a battle of magic and loyalty within the Potter family and among Harry's friends.
Another place to check is FanFiction.net. There are many Harry Potter fanfictions there as well. Browse through the Harry Potter section and look for tags or summaries that mention Eileen Prince. However, the quality may vary, so you might need to read a few to find the really good ones.
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.
Maybe there could be a new antagonist, a dark shapeshifter who is jealous of Harry's power. This antagonist could be an old wizard who has been in hiding for centuries, waiting for the right time to emerge and take over the wizarding world. He sees Harry as a threat to his plans and tries to eliminate him.
Another possibility is a corrupted member of the Ministry of Magic. They might be trying to use Harry as a scapegoat to gain more power within the Ministry. They could be working with the Death Eaters or acting on their own, but they would be a significant obstacle for Harry as he tries to stay alive and free while on the run.
Perhaps a new dark wizard or witch who has infiltrated the Ministry of Magic and made sure Harry was sent to Azkaban. This antagonist could be working towards a bigger plan to take over the wizarding world and sees Harry as a major threat even when he's locked up.
It could be Voldemort. He might see Harry's new waterbending ability as a threat and try to find a way to counter it. Maybe he would seek out dark magic related to fire to oppose Harry's water powers.