Great work as always. If you are still looking for ideas I have two that I thought of because of the gymnasts:

1. Split between two objects, just a foot/shin on each. A gymnast girl should be able to oversplit that pretty easily. But then one could add weights, and then pick them up and drop them with the weights so they split wider. At some point either their hips will dislocate or their legs will break, either way they will fall off of it (unless their feet are tied down). One could put a spike underneath.

2. A tippie toe squat ( see reference below ), except with something under their asshole ( I would prefer a beer or wine bottle ) and a strip of nails under their heels. And then add weight, either the bottle goes in, the nails go in, or both.

https://xhamster.com/photos/gallery/9744196/187423276