Sounds great! Just to be clear on my last point about sliders, I guess I'm talking about an additional "slider constraint", that automatically moves the slider position based on a chosen bone property and range. While manually keying the slider position is fine, I really want something linked to existing properties that doesn't have to be keyed at all. For things like physics, it makes sense that you don't want to have to meticulously match a curve to what it's doing automatically.
For more, unnecessarily detailed info: 😅
The end goal for my turnaround example above would be similar to how isometric setups work currently, with an x-scaled bone parenting a rotatable bone to get oval-shaped rotation. You can then make a bunch of these oval-like joints centered on a rotation axis, to position every joint by its distance and angle from that central axis, defining your character in a 2.5d/polar coordinate way. By then linking the rotation offset of the child bones to a master "rotation" control bone that offsets them all, you can animate something like a walk cycle once and get a full 360 degrees of walking animations for free. In code you can just change the rotation of one bone via transform constraints to turn your character to the correct direction, and all the ovals swivel to that offset. But most importantly, if you want arm rotation to be decoupled from the body for better animations, you can just nest 2 control bones via transform constraints to get a "master" controller and an "arm" controller.
This rig setup mostly works in Spine right now, the only problem is that the art/attachment layers don't understand the concept of a control bone or the transform constraint, so draw order is static and also art frames have to be manually swapped out. Currently I can make a separate turnaround animation that has the correct draw order and art frames at every angle for every piece that rotates independently, then blend them in code by freezing animation layers and manually setting them to the correct turnaround point, so it somewhat works in runtime code. In Spine though, this is impossible, so I'd have to check all my animation angles post-export to do any kind of cleanup.
I feel like there are a lot of use cases where these kinds of art control bones would be helpful to the animation process. So yeah, not really a core feature, but very powerful in certain circumstances, like with physics and "3d" rigs. I feel like this is a situation where a picture is worth a thousand words though, so I'm going to stop rambling, haha.
