I know, I'm doubling up suggestions, but I think these'd both be pretty beneficial:

Shaft angle: this is already semi-hacked-in on a few characters by tying shaft angle to the "growth" slider(which, iirc, the tooltip specifically tells you not to do lol), indicating that a decent number of creators already want it as a feature. Not having it modifiable outside of the character creator is one of the larger glaring issues I've seen while making interactions, as there doesn't seem to be any single shaft angle that's readily compatible with all penetrating positions and the "angle" property in couplings is a bit too restrictive. At this point I've been getting around it by forgoing the animation component of the coupling system altogether and using custom timelines for a lot of positions, which comes with its own caveats. There's also a lot of situations outside of using a coupling where you might want to change the shaft angle, such as characters grabbing and interacting with a shaft, or angling it to a more appropriate position to start penetration.

On top of this, the shaft angle for some characters (ones made before interactions, particularly) can be unusable for an interaction and require a new export and upload of the character to fix.

Modifying part stiffness in animations: would be useful in making characters' tails more animated. Many animals don't have permanently stiff, permanently relaxed or permanently raised/lowered tails. I noticed this with my recent wolf character: I set the stiffness on his tail to be roughly how a wolf's tail would be when relaxed, but now it gets in the way in some poses and if I want him to wag or raise his tail there's currently no way to do that. Ears are also a bodypart on certain species or breeds of animal that aren't permanently floppy or perky, and there's other more inventive uses for this I'm sure.