Voldemort can be a common character in such fanfics. If Harry dies, it might be at the hands of Voldemort or related to the fight against him. And Dumbledore, even though he might be dead in the original series, his influence and legacy could still be a part of the 'Harry Death Fanfic'. His wisdom might be something that the other characters reflect on after Harry's death.
Neville Longbottom could be a significant character. He has always been a strong and brave character. In a 'Harry Death Fanfic', he could step up in the absence of Harry. For example, if Harry dies before fully defeating Voldemort, Neville might take on the mantle and lead the fight, using the inspiration he got from Harry while he was alive.