Amateur
Asian
Twinks
Latino
Euro
Bisexual (note: many guys won't stay on a site showing women, use this with caution - if you personally push a site like 3 pillows, you might want the option)
Ebony
Uncut
BDSM - yes it's a niche, but a huge one and almost demands a category listing - or leave it for webmaster to put into general Fetish
Bears
Pornstars/Celebrities
Fetish (this would be the blanket term for smaller targeted fetish niches like watersports, handball, balloonsex, plushie, diapergays, foot fetish, and so on)
Jackoff (single or more doing masturbations, covers solo male sites and hunks, studs, jocks in single presentation)
Mature
Oral/Anal - can just list as Hardcore since they usually show both in a set
Outdoors
Group Sex
TransSexual
Uniform (applies to military, work clothes, suits etc)
Voyeur

May of missed a couple, but that's a start!