If Harry joined another house in fanfiction, the story's plot would shift significantly. In a way, he'd be an outsider within Hogwarts in a new sense. His friendships would be remolded. For instance, Ron and Hermione might not be as close to him if he were in a different house. And the way he approaches challenges would change. Gryffindors are known for their boldness, but in another house like Ravenclaw, he might approach things more analytically. His enemies might also view him differently. Malfoy, for example, would have a different kind of rivalry with him if he were not a Gryffindor.
In fanfiction where Harry joins another house, one big change could be in the Quidditch matches. If he were in a different house, he'd be playing against Gryffindor instead of for them. This would change the whole dynamic of the matches. Also, his relationships with the house ghosts would be different. Each house has its own ghost, and being part of a new house would mean interacting with a different spirit. It could also mean he'd have different dormitory mates, and their influence on him could be quite significant.
He would likely be more humble. House elves are known for their selfless service, and growing up with them would probably make Harry less self - centered. He would understand the value of hard work and helping others without expecting anything in return.
His personality would be significantly different. Since house - elves have a very different culture from wizards, Harry would grow up with different values. He could be more empathetic towards the underprivileged in the wizarding world, like the house - elves themselves. He might also be less concerned with the pure - blood/muggle - born divide, as house - elves don't really care about such things. Additionally, he would have a strong sense of duty towards those he considers family, which in this case would be the house - elf that raised him. This sense of family and loyalty would be a core part of his personality.
The story would change in multiple ways if Harry joined. Firstly, the location aspect. Harry is used to Hogwarts and the wizarding world locations. But in the Slenderman world, it's often in creepy forests or abandoned places. He would have to adapt to these new settings. Secondly, the conflict. Instead of the usual battles against Death Eaters or dark wizards, he would be facing a completely different kind of threat in Slenderman. His magic might not work in the same way as it does in his own world. He may need to find new ways to use it or combine it with the resources available in the Slenderman world. Also, the relationships in the story would be different. Harry might form new alliances with characters in the Slenderman world, or there could be conflicts between his values and the way things work in that world.
The story might start with Harry being somehow transported to Vader's universe. He'd be shocked by the new environment, but his quick thinking would help him adapt. Then he'd cross paths with Vader, and the adventure would begin.
Voldemort would also change in this scenario. He might be more lenient with Harry at first, seeing him as a valuable asset. He could start to show a side of himself that was more paternal or mentoring towards Harry, which is a far cry from his pure - evil self in the original series.
The story would become much darker. Bellatrix is a symbol of pure evil in the Potter universe.
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.
Well, it would be a lot less antagonistic. Without the whole Death Eater past and the spying for Dumbledore while seeming to be on Voldemort's side, Snape wouldn't have that built - up resentment towards Harry. He might actually see Harry as a talented young wizard worthy of proper mentorship, instead of constantly being so harsh on him.
He'd likely be more humble. House elves are humble creatures, and growing up with them would instill those values in Harry. He wouldn't be as boastful as he sometimes is in the original series.