That's pretty cool!
When it comes to making rigs for flexible limbs, there's a whole lot of 'it depends.' The two main ones are, 'how do you want to control the limb' and 'what kind of animation do you want to do.'
Moving the limb around to interact with objects, grab people, express emotion, is different from trying to get a tentacle to slide smoothly, such as when being used as a toy. For the most part, I wouldn't worry about trying to get a tentacle to slide around, since setting that up typically is dependent on what particular animation an artist is trying to achieve, so they have to customize it every time regardless.
So then the rig depends on your philosophy for how the limbs should move and be adjusted. Should they act like arms, with an elbow like bend that can be controlled to bend down or off to the side. Should the limbs act like they have N joints since technically it is just a tentacle that can move however it wants and not look like an arm. Should the limb squash and stretch? How should bending be handled, should the mid length area be large radius bends and look smooth, and the end of the tentical more crisp and sharp so that the animator can have it finely grab things?
I'd also maybe ask about a way to control different ways the limb behaves, but in my experimenting I haven't figured out a good way to do this, and I don't play with blender code enough to add more complex behaviors. I also haven't experimented much with manipulating curves and having curves manipulate limbs, but I know that there are a few techniques around the idea of having a tentacle fallow a curve.
If it were me, I'd try to combine 4 techniques:
- Have an IK control a 4 bone rig, to allow for bending and posing of the limb, with a manipulator rig at the end to allow for fine control of the tip.
- Use the 4 bone rig to manipulate and bend a curve to control how the limbs smoothly bend rather than looking to have 4 bones (this is the one I'm not sure how to do, idk if curves can even be manipulated by bones this way)
- Have a length control node that slides the limb across the curve, to allow the limb to slither or at least act soft and squishy (volume is preserved) by allowing it to move in and out of poses.
- And a control path for the IK to allow for temporal planning of animation paths, possibly with the control curve also be controlled by objects of interest (this way a target character can move and bounce around without having to animate the limb to follow, it will do this automatically while smoothing out its own bend paths)
If you wanted to look into this, simply learning about IK's and using curves to control bones is where I would start, beyond that, its just experimental and experience.
Beyond that, it looks like you have a good start with the idle animation, with the only improvement I could think of being 'secondary physics' to make the tentacles bob and swing a little out of sync and organically, but that may just be a style choice.
