Looking for BL or yaoi where either the hero falls for the villain, or the villain falls for the hero. A one-sided unrequited love with action & even better if it has combat, like a gang war, D&D style battles, martial arts, or Wuxia style magic fighting.

Yuusha in Maou n Chi has hero/villain, but it's a gag/parody manga and therefore probably not what you're looking for.

This kind of theme is very rare in manga, so I think looking to BL novels, especially Chinese BL novels, would yield more results. Novel Updates is the novel equivalent to MU and if you search through its database you'll likely find plenty of titles that fit your requirements.