I'm in the middle of writing a fan fiction where a character who knows about the events of Fate/Stay Night (and Fate/Zero) is thrown into Fuyuki City, three weeks before the 5th grail war begins. It basically started out as a self-insert series, though I altered the main character a bit and changed quite a few things from my original plan. Other than the MC, I also have an OC Servant, who replaces one of the Servants of the Grail War, though I won't spoil who (not really that big of a spoiler anyway, but w/e).
I wouldn't consider myself a good writer by any means, but I still think it is semi-decent.
Here is a link to the story. If you don't like the prologue I'd suggest reading the first chapter, because the prologue is probably the worst chapter I've written. I'd honestly consider removing it if it weren't for the fact that it is kinda important in setting the relationship between the MC and Rin (no, this doesn't mean a romantic relationship).