The plot might develop from a prophecy. A prophecy in the Harry Potter world predicts a great danger that can only be averted by the combined forces of both worlds. Characters from both universes are then drawn together. They initially struggle to trust each other due to their different ways of life and values. However, as they face the first signs of the impending doom, they realize that they have to put aside their differences. They start to train together, sharing their knowledge and skills. As they get closer to the source of the danger, they discover that an old enemy has found a way to merge the two worlds in a chaotic and destructive way, and they must race against time to stop it.
The plot could start with a mysterious portal opening between the two worlds. A Hogwarts student might accidentally stumble through it into the world of Strike the Blood. This would lead to a series of adventures as they try to find their way back home. Along the way, they could meet the characters from Strike the Blood and learn about their world. The student could also introduce some basic magic concepts to them. As news of this strange visitor spreads, it could attract the attention of both the good and bad forces in the Strike the Blood world, creating a lot of tension and action.
A possible plot development could be centered around a powerful magical artifact. In the Harry Potter world, an ancient and powerful relic has been stolen and somehow ends up in the world of Strike the Blood. A group of Hogwarts students, along with some of the Strike the Blood characters, must work together to retrieve it. They face many challenges as they have to deal with different cultures, magic systems, and power - hungry villains who also want the artifact for themselves. As they progress, they start to understand and respect each other's worlds more, leading to new friendships and alliances.