The story would change in many ways. Firstly, the power dynamics would shift. Harry has his own set of powerful magic, which could tip the balance in battles. Secondly, the relationships within the group would be different. Harry might have a different approach to problem - solving compared to Rias's group members. So, there could be some friction at first but also a lot of growth as they learn to work together.
The story would change significantly if Harry Potter joins Rias Gremory in fanfiction. Harry's moral compass, which was shaped by his experiences at Hogwarts, would influence the group's decisions. His ability to handle dangerous magical artifacts could be crucial in Rias's world. For instance, if there were a situation similar to the Horcruxes in Rias's world, Harry would know how to deal with it. His presence would also likely bring a different tone to the story, perhaps a more heroic and self - sacrificing one, as he is known for putting himself in harm's way to protect others. This would contrast and blend with Rias's own style of leadership and decision - making.
If Harry Potter were to join Rias Gremory in fanfiction, the story's scope would expand greatly. Harry comes from a world full of mystery and unique magic. His presence would attract new enemies and allies alike. His knowledge of the Deathly Hallows, for example, could introduce a whole new aspect to the story. Also, his friendship with Ron and Hermione might be a factor. Maybe they could also get involved in some way, further complicating and enriching the story. The story would no longer be just about Rias's world but a blend of two very different magical universes.