One of the biggest problems I see with Yiffalicious right now is that there's (as far as I can tell) no real way to curve a character's spine. You can make characters bend at the hips with the spine anchor, but the spine itself stays ramrod-stiff.
This creates problems when trying to do things like create a missionary scene. Realistically, kneeling and thrusting in the missionary position, your knees themselves will be stationary. Yiffalicious currently doesn't have a way to simulate this behavior. As a result, the knees wobble and don't give the illusion that they're supporting the character's weight. In the worst case, they clip through the surface they're supposedly resting on.
You run into this anytime a character is kneeling and thrusting (or sometimes being thrusted against), as well as sometimes when characters are resting on their elbows or shoulders.
One workaround for this issue would be the ability to establish unmoving joint anchors somewhat like we can already do with extremities. This wouldn't look especially realistic, though, and I know you guys are not real fond of "workarounds."
Let me know if you need images or gifs or something demonstrating what I'm talking about.