The main antagonists could be the top - ranking officials in the Ministry who are desperate to hold on to their power. They would likely use all means at their disposal, including dark magic and underhanded tactics, to stop Harry and his allies.
There might also be some Death Eaters who have infiltrated the Ministry and are working with the corrupt officials. They see Harry's attempt to overthrow the Ministry as a threat to their own goals of taking over the wizarding world, so they oppose him.
There could be a group of dark wizards who are secretly operating within the Ministry itself. They oppose Harry's alliance with the Ministry because they fear that Harry will expose their illegal and immoral activities, like illegal potion - making or dark - magic experiments.
Another plotline could be that Harry, through some newfound powerful magic or an ancient prophecy, is seen as the rightful leader to bring about change in the wizarding world. This gives him the support of many wizards and witches who are tired of the Ministry's ineffective or unjust rule, and he uses this support to stage a revolution and overthrow the Ministry.
A group of jealous students could be the main antagonists. They might be envious of Harry's natural talent and the attention he gets from the teachers. So, they try to sabotage his progress at the gifted school by setting him up to get in trouble or stealing his study materials.
Maybe a group of anti - magic fanatics within SHIELD itself. They see Harry's magic as a threat and try to undermine him at every turn. They could be secretly working with external forces that also oppose the use of magic in the world of SHIELD. Harry would have to prove that his magic can be an asset rather than a liability.
It could be a group of dark wizards who don't want Harry to succeed in becoming a cursebreaker. They might be using dark curses themselves to cause chaos in the wizarding world, and they see Harry as a threat to their plans. So they try to stop him at every turn, whether it's by setting traps or sending out cursed creatures after him.
Maybe there are some ancient dark wizards that Iskandr has a history with. These wizards could see Harry as a threat because of his potential, especially with Iskandr's training. They might emerge from hiding to try and stop Harry from reaching his full power, fearing that he and Iskandr together could disrupt their long - planned evil schemes.
There could also be a new group of supernatural beings, perhaps a cult that worships dark magic. They might target Hellsing because of their power and the fact that Harry, with his wizarding abilities, is now part of the organization. This cult could be trying to summon some ancient and extremely dangerous entity, and Hellsing, with Harry's help, has to stop them.
There could be a secret society within the wizarding world that has long opposed the Knight Templars. They see Harry's new - found connection to them as a revival of an old threat. They try to undermine Harry and his attempts to bring the Templar values into the wizarding world by spreading false rumors and creating obstacles for him at every turn.
Some pure - blood wizarding families who value strict traditions and order might oppose Harry. They might view his very existence as an affront to their way of life. They could try to use their influence to drive him out of the wizarding community or plot against him in secret.
There could also be a group of dark mages who want to use the chaos of this inter - dimensional relationship for their own gain. They might try to manipulate Harry and Serafall's families and friends against the marriage, and then when things are at their worst, try to steal their combined magical power which they believe will be extremely potent when Harry and Serafall are united through marriage.