Why can't we have one rig to fit all purposes and to have it turn 360 smoothly?
I believe the answer is rather simple since rotating a simple skeleton 360, in Moho, is not that complicated - most likely you'll have more than 1 character in your animation and most certainly you'd prefer to have some differences among them.
But it still doesn't answer why can't we have one basic rig that can be modified for other characters.
The answer is partially there, in the previous one. You'd most likely prefer to have something specific for your take on the character. T-pose is already invented and everything else is on you. So...
How many limbs does your character have?
How it is dressed?
How detailed do you need its outfit to fold/unfold?
Bold or hairy?
Glasses? Transparent or sunglasses? Monocle, perhaps?
Is there a cape?
And so on...
In short, even in movies, there's no single actor that can play all the roles. It has been done by several actors but if you think of those movies, there must be one or two in which these actors didn't (couldn't) play all the roles that well. Despite the masks, CGI or some other "helpers", there was something odd about it. I believe the same could apply in this quest for a rig to fit 'em all.
However, the most useful tip/s I could find on building a universal rig is in this tutorial for Animate CC (link).
The same author also made (an attempt for) such a Multi-view rig in Moho (link).
Having a simple or basic shape such as a circle (I'd add a square or vertical rectangle as options as well) at the center, and as a base of your character build up, can resolve a lot of doubts about the universal rig. Adapting the character design to such a simplified shape could simplify the rest of the expectations you'd have for your character (design) to meet.
In dancing and most martial arts (i.e.), the matter of body center is crucial for proper performance. Being aware of your center is important in keeping the balance that enables you to properly perform the desired actions. As expected, you'd shift your center in all directions but positioning your limbs and torso to accommodate the new posture would require practice and eventually produce a skillful performance. Further on, since we all differ in this or that matter, how come different people can perform the same skill but the same could not be applied to rigs? Well, I haven't heard of a sumo wrestler who excels in tango performances, although, I've seen some corpulent people doing it rather gracefully. But shaping up your character design from its center should be a solid and reliable guide for the rest of the process - the character design and for multi-view rig.
If you have in mind such preserving of the balance/center of your character, while performing on the scene, after a while - you should eventually have all the questions and answers. Since you'll have to resolve the main "joint" of your character, the same must affect the following (child b)ones...
The things which instantly came to my mind, about the rig structure, would be...
Is this a limber or jugged character?
Lightweight, heavy or massive?
Do I need shoulders for this rig?
If it's two-legged, what kind of groin depiction would be the most efficient?
Do I need hips as well or just legs attached to the spine/root bone?
Would there be 1 or more bones in the spine?
Is there a tail?
... for starters.
One could say, well there you have it, narrowed to 2 types - with or without shoulders & hips. Enough to build The Universal Rig for each.
Sorry, but I can't help it - I'll always root for diversity. Any sort of simplification and universality brings a reduction as a main tool so, in my conclusion, I'd say the matter of universal rig lies between Deductive and Inductive reasoning. Since we still haven't invented a reliable compromise between these two - not much chance left for the universal rig, I'd say
