I would advise you to look here first:
Type-Moon Fanfic Idea Thread
Miscellaneous Fanfiction Writing Thread (Solicit Advice Here)
Questions, questions and more questions
The fanclubs have generally devolved into a place to post fanart. You won't get much help here.
Tobias wrote a fic called Dear Tohsaka, that's basically a letter to Rin from Sakura, with two sequels. It's quite sad, but very good, and characterizes Sakura very well. That's the overall consensus at least: Dear Tohsaka
I've done something like this myself. If you want to have a look at it, though it's not written in the first person: Letters From Tohsaka: Fuyuki
The title is similar, but it was a coincidence due to some similar subject matter.