Spring physics for interactions
Having springs would be a great way to add some liveliness to the way characters animate while coupled, instead of their bones either being statically placed or strictly following their parent/timeline position.
Yiffalicious had this in the form of limb springs, but in FVNE this too could be taken further.
Mouth point IK is a good example, right now it causes the character's head to follow it's location exactly - good for timeline animations, less so for coupled thrusting.
What if, instead of a mouth point, you could add an empty node, place it where you want your character's head to be, and then add a spring. The spring would let you pick two nodes to connect, set a spring constant (stiffness) and an amount of dampening. The nodes would then be pulled towards each other (an empty node might be static, or maybe any node could be toggled to be). In our example the characters head would now be pulled back and forth around the point we want, instead of sticking to it in a static manner.
This kind of system might also help make the way characters push each other in coupled thrusting feel more weighty. Right now the receiving character of "push beyond" force seems to accelerate instantly and stay glued to the pusher - something that could be improved for sure, and maybe a spring system might be a way to do it?
I was going to suggest some sort of "inertia" on push beyond because it seems like it drags the thruster along with it, while it should be more a contact and bounce away motion, i.e. the pushed character moves forward away from the thrusting character while the thrusting character starts moving backward.
This would necessitate treating the penetrator as being temporarily disconnected from the orifice on the backswing I suppose. Would probably need pull resistance set to 0 and act like it does now if it was > 0.
Push beyond looks weird and jerky.