I would personally love to see a female servant who uses her breasts much like Vaal does her ass. She would work well with pitfall traps, providing a soft landing for the Hero in her cleavage, but like Vaal, would squeeze the Hero repeatedly to drain morale. And, also like Vaal, if you let her eat the Hero, her boobs would grow.
I also agree with the idea of servants becoming more capable the more they're fed, perhaps changing how some traps work. And as they level up, they get bigger.
Trap ideas include a liquid one for slipping, possibly falling into a previous trap. A sticky trap that makes the hero more vulnerable to your servant. And a net/web trap to also hinder the hero's movement.