Original plots are, of course, preferred. If you do take a classic or overused plot (in the case of fanfiction, you'd mostly be talking about the ingame storylines here,) you need to really put your all into the other aspects of the story, such as characterization and such. It's also helpful if you add subplots and the like to distract readers from the fact that the main piece is something they've seen a bazillion times before.
Personally, I'm a fan of really twisty-turny plots. I love books where you go through the whole thing thinking you understand what's going on, before the tables are suddenly turned and you realize that things have changed and nothing was what it seemed. Of course, you don't have to go all crazy like that, but really, predictable plots are the most boring to me.