Great framework, will love to see what it becomes.
As for fetishes, definitely Tf given the anatomical description already in place.
Feminization/gender bend depending on what gender you pick in the beginning.
More through sex descriptions and actions, with detailed anatomy.
Prostate milking/play, and different kinds of orgasm depending on what kind of stimulation is received.
Chastity/orgasm denial, with armor/clothes to go with it.
Sex toys that can be equipped in place of a weapon, and usable in both sex and battle.
A blending of the current sex and battle systems, maybe making them one and the same?
Alongside the current slit penetration option, more unusual/fetishistic penetrations, such as sheath, urethral, etc.