The Death Eaters could still be great antagonists. They are used to facing mages, but when Harry shows up as a mage - warrior, they are thrown off. Their dark magic might not be as effective against Harry's combined skills. For instance, Harry could use his warrior reflexes to dodge their curses and then counter with a powerful mage spell.
Even some of the more traditional pure - blood wizards who look down on the idea of a mage - warrior could be antagonists. They see Harry's abilities as an abomination and try to stop him from using them. They might try to use their influence in the Ministry of Magic to get Harry in trouble or even try to harm him directly.
The antagonists could be a group of corrupted Ministry of Magic officials. They have turned to the dark side for power and are trying to rewrite the laws to suit their evil plans. Harry, being the hero, sings 'This is War' to expose their wrongdoings and fight against their tyranny.
Maybe there's a powerful and corrupt wizard in the Ministry of Magic who doesn't want Harry to have an independent means of travel like the yacht. This wizard could send his minions to disrupt Harry's work, or even create false laws that would make building the yacht illegal in the wizarding world.
The Death Eaters could be the antagonists. Even though Voldemort is gone, some of his followers might not want Harry to succeed in building a starship as they fear it could bring more power to Harry and his friends, and they still hold grudges against him.
One possible antagonist could be a corrupt wizard in a high - ranking position in the Ministry of Magic's financial department. This wizard might be trying to cover up his misdeeds and would see Harry as a threat to his scheme.
The Thalmor could be great antagonists. Their strict control over magic and their anti - human beliefs could put them at odds with Harry. They might see his different form of magic as a threat to their power in Skyrim.
It could be a new dark wizard who wants to make a name for himself. He sees Hogsmeade as an easy target to show his power. He starts by causing small disruptions like making objects in the shops move on their own and then escalates to more dangerous attacks that Harry has to stop.
Some pure - blood supremacist wizards might oppose Harry. They may not like the idea of a half - blood like Harry having so much control over a city and could try to create unrest. Also, there could be some non - magical criminals who target the city because of its new magical elements under Harry's leadership.
The Ministry of Magic could be a major antagonist. Since necromancy is considered a dark art, they would try to stop Harry at all costs. They might send Aurors after him or try to expel him from Hogwarts.
Scarecrow could also be a main antagonist. His fear - inducing toxins could be a real problem for Harry. Harry would have to use his magic to counter the effects of the fear toxin and also figure out a way to stop Scarecrow from spreading it all over Gotham.
One great 'Harry Potter Warrior Mage fanfiction' is 'The Mage's Ascent'. It features a young wizard who decides to train as a warrior mage, adding a new and exciting twist to the familiar Harry Potter world. The story has detailed descriptions of magical battles and the character's growth in both magic and combat skills.