One possible plotline could be that Harry discovers his connection to Buffy and has to navigate the world of slaying while also dealing with his own magical world in Harry Potter. Maybe he uses his magic to help Buffy in her battles against vampires and demons in a way that no one else can.
One possible plotline could be that Harry grows up with a stronger family bond and more knowledge about his family's past. He might be trained in magic earlier and more effectively by James. Another could be that he is more involved in the fight against Voldemort from a very young age, as he is part of the Potter family that Voldemort is targeting.
Maybe the fanfic could focus on the brother's backstory. He might have been bullied as a child because of his family name or his lack of a 'powerful' quirk at first. This could lead to him developing a strong sense of justice and a desire to protect those who are weak. His journey from a bullied kid to a confident hero - in - training would make for an interesting story.
One plotline could be that they discover their relation later in life. Harry, having grown up in the wizarding world, and Superman, in his superhero world, suddenly find out they are brothers. They then have to combine their powers and knowledge to face a common enemy that threatens both their worlds, like a powerful dark wizard who has also got hold of some kryptonite - like substance.
One possible plotline could be that they discover their relation later in life. Harry, being used to the magical world, is shocked to find out he has a brother in a completely different scientific realm. And Bruce has to come to terms with having a wizard for a brother. They then start to learn from each other's worlds, with Harry showing Bruce some basic magic and Bruce trying to explain his scientific research to Harry.
One possible plotline could be Cedric helping Harry train for the Triwizard Tournament. Another could be them uncovering a mystery together at Hogwarts.
One plotline could be Buffy being sent back in time to the Korean War and having to fight both the normal human enemies of the war and some supernatural threat that has emerged because of the chaos of the war. Another could be that a vampire or demon from the Buffyverse somehow gets involved in the war on the side of one of the combatants.
One possible plotline could be that the kiss is a moment of unexpected passion. Maybe they've been friends or rivals for a long time, and the kiss changes everything. Another could be that it's part of a magical spell gone wrong, where they have to deal with the consequences of this unplanned kiss. It could also be a plotline where the kiss is a secret moment that they both try to hide from others at first.
There are many fanfic communities out there. Some popular ones might have the 'harry is buffy's brother fanfic' you're looking for. You could also ask in relevant forums or groups dedicated to either the Harry Potter or Buffy the Vampire Slayer fandoms. Fellow fans might be able to point you in the right direction. For example, Reddit has some great sub - forums for these types of things.
One possible plotline could be that Harry discovers he has a half - brother he never knew about. Maybe his father had a relationship before he met Harry's mother, and this new character enters Harry's life. They initially have some conflicts due to jealousy or differences in upbringing, but over time, they come to love and support each other.
A possible plotline could be that Draco, out of curiosity, starts to research Harry Potter's mother. As he delves deeper, he finds out about her selfless love and her powerful magic. This makes him question his own values and beliefs, leading to a transformation in his character.