Hmm. Well, my very favourite is things with power dynamics. Slavefics, fics where someone's royalty, D/s, anything where one person has power over the other. I've seen very little of it with f/f... in fact I can only think of one original fic & one published book that fit, at least that I actually enjoyed.

I like many other types of fics too, especially scifi/fantasy, but they tend to be plot-focused, rather than relationship-focused. I may be looking in the wrong places, but I've seen little femslash that qualifies.