I think I would have liked that Sirius Black survived.
After all those years in Azkaban and then on the run, it would have been deserving to have him as Harry's guardian.
I would surely change the stereotyping of Slytherins as evil and make the final fight between Voldemort and Harry more intense and longer as in the whole series we are waiting for such an encounter but it ends too soon and leaves one hungry for more