I tend to only offer fandoms where a) I already have source in-hand and b) the thought of spending a bunch of time with it makes me smile instead of feeling dread.

And as the years go by (here and also in Yuletide) I find I'm less inclined to either offer or request a fandom if I've received something amazing in it already. So, e.g., last year I got amazing Big Eden vids, and this year I don't feel the need to offer it or to request it -- my niche is filled.