Mm hm...after all, as his river was destroyed, Haku had no purpose as a spirit anymore, did he? That would be a good follow up movie, what with Chihiro tying to gain his friendship (he could be quite cold and mean - personality chnage as well from the thing/person?) and trying to make him remember.
He could slowly become nicer, like protecting her from bullies or something, and then at the end, regain his memories...
But about the original movie...don't you think that the soot sprites are adorable?