In both cases, defying those expectations is possible, but it's more work, and it's not often done because it will disappoint or anger a good chunk of the readers. Some writers wind up "filing off the serial numbers" so that the fanfic is no longer recognizable as such (or is recognizable in a camouflaged, wink-and-a-nod way). You end up with a starship that isn't quite the Enterprise, or a powerful life-draining immortal who isn't quite a vampire. There has been some quite good fiction produced in this way.
Although I recognize that fanfic is a useful writing exercise, and can be used as the basis of some interesting transformational works that take the basic material and comment on it, or use it to comment on other aspects of society, I generally find it uninteresting because it's lazy. And that might be why I find so much vampire and zombie fiction (and there is so much of it, these days) extremely put-downable.