I have to say that it is way easier to write a relationship fic than a case-related fic. I know there are some VERY TALENTED writers out there that can write a fic that is incredibly similar to an episode. But, I also think that when a couple is together either on the show or in our imaginations it's easy to come up with the logical next steps. Things that aren't on the show, such as; a marriage proposal, moving in together or even a first date. I think it's easier for fanfic writers to imagine those types of scenarios rather than try to write something that is case/crime related.
Forgive me if I offend anyone, it is not intended, but are most of you women? I'm quite happy to spend my time reading case-file fics but the relationship stuff just seems to be like CSI: Erotica rather than anything else. I spend so much time writing my Star Trek fanfic and my novel, I don't have time to develop a CSI fan fic, though I promise I will when I get the time (even if it means writing less of the ST fan fic for a time).