In this fanfiction, it could start with a chance encounter. Harry could be lost in a magical forest near Bill's place. Bill finds him and takes him home. There, Harry gets to know Bill's family better. They form a bond over their shared love for magic and stories of the wizarding world. As they grow up, they continue to support each other through different challenges at Hogwarts and in the wizarding community at large.
Well, perhaps they could become friends quickly. Bill might introduce Harry to some interesting magical things he hasn't seen before, like some unique creatures. And they could start having little adventures together around the wizarding world.