I've always had a thing for girls doing gymnastics. Nice, soft but toned bodies, and enough fitness, strength and stamina to endure a fair bit of torture. It also opens up the possibility of having different ages at play, with different experiences with and reactions to the sex and perverted torture happening in the dungeon. I'm sure however, that whatever you end up writing will be very enjoyable (by us and the trio that is, not so much for the girls…)