I concur that twin sisters (or triplets) is definitely sexy on the taboo side. In fact, I know a guy who was able to live that fantasy, although the twins never slept with him after that -- they did still hang around the group though.
My fantasies are probably on the more extreme end, not the most extreme, but up there:
Violence,
rape,
blackmail / extortion / homewrecking / life-ruining,
giving and receiving human toilet,
being made to eat filth from dirty feet,
public sorts of things
Most of the time I like to be the "victim", although I actually prefer to be more of a chad in the wild. (I'm not a chad, but I like to try and be)