Over9000GHz said:
impregnation_request is my personal underrated favorite. Something about her asking/forcing me to dump my cum into her sends shivers down my spine, and makes me 9000X harder.
Seconding this. There's so much about a girl begging me to get her pregnant that makes sex that much more hot.
Of course, everything after the fact (pregnancy, lactation, birth, etc) is incredibly hot to me too, though I'm not particularly inclined to say they're "underused". I'm more inclined to say "enthusiastic_consent" is one I rarely see, but always love.