trampling is a niche on it's own that is not a subniche of anything else, but related to feet and\or boots and\or shoes and\or femdom.
do you see how complicated this is? no person's sexuality is the same as the next.
on one foot fetish directory I have I put them a trampling/crushing/GTS category:
http://www.fruityfoot.net/Trampling-Crushing-GTS.php
on a femdom toplist, I put them in a unique trampling category:
http://www.dominant-woman.com/Trampling.php