For me pregnancy is certainly a bone killer, just the though of pregnancy... But alternatively, lactation is hot. Not sure why. But breastfeeding as an adult is a turn on.
Hardest: watersports, but more like toilet slavery with swallowing the urine. It pairs with femdom, where you get treated like a toilet to your domme. I think it's the loss of control, the objectification, the fact that you're treated as something so demeaning.
Breath play too. Usually smothered by a breast or ass, but hanging by a noose can be a turn on.
Wierdest: giantess, more specifically being shrunk to say 5 inches to an inch tall. Then being treated like a pet and/or toy to the giantess. That feeling of being helpless, the loss of control (again), and it sounds fun to ride around in a pair of panties or bra.
Then there's transformation, into actual inanimate objects, usually something sexy like a pair of panties, a bra, a dildo, or even other objects that a woman might use. Clothing, lipstick, even a glass of wine.
I think all of the above is about losing control, and being treated like you're nothing. Not sure why it's such a turn on.