As for writing for other fandoms, I just can't imagine why anyone would ever consider it <g>!
Actually, the first fanfic I had the guts to post on the internet was in the Forever Knight fandom. If not for the nice folks in that fandom and their encouragement, I'd have been too chicken to try writing anything else and posting it online.
I tend to concentrate on one fandom for a while, then try writing in another fandom when my muse is blocked. It helps me to stay on my mental toes. I only write in fandoms where I really know most of the characters, though.