it'd be interesting to see what kind of rig you'd come up with for the mouth/snout. That's one big piece that I haven't been able to do nicely with Papa Bear.
I've yet to come up with a rigging approach I like for a fully articulate... and highly expressive... exaggeration-ready... mouth but...
As far as I can ascertain there are two basic needs for rigging of a mouth. The first is the jaw (rotation and lowering) to get at the basic shape of an open mouth that can stretch while still maintaining a since of the solid jaw underneath. The second is a ring of bones around the mouth to provide the shapes necessary for lipsync/dialogue.
The most successful tests I've come up with only have the jaw bone and the mouth shapes are manually animated. This matches the general approach of lipsync with phonemes ala the Dopesheet/poses.
Two areas of concern seem to be most prevalent when modeling the lower face and the first is that the mouth needs to wrap around to the sides of the face at least enough to gain the ability to pose the mouth in front, side and (perhaps most importantly) three quarter views to aid in the viewer actually seeing the shapes.
My own goal is to make sure the mouth can be moved into extreme positions beyond simple dialogue shapes in order to target and really push expressive emotions. When drawing these extremes can be pretty easy to achieve but with a rigged character the animator may find they've hit a limit that they can't move past that they require to get the shot they want.
I've found a useful approach to modeling the mouth is to model it in an extreme open position first, to more easily have access to all the control points, then adjust the jaw/mouth/lips back into a proper position for a relaxed state. I can't say that I've achieved success with this yet but I've failed enough in the attempt to know what doesn't work so well.
I can see where Papa Bear's mouth might prove troublesome.
Like most articulate faces I would think the snout would be maintained with the upper skull (and move very little) and the mouth follow the jaw.as it rotates and moves. The skinning/mesh between skull and jaw then needs to be able to accommodate the resulting squash and stretch.when the jaw's movement forces changes of shape.
I can't envision how you got from a flat plain to the 3D head.
As with most things I always think I'll record a session and share it but I haven't quite got to that point yet.
I will say that it might be well worth the time to model the skull area and the jaw area separately and then later stitch them together.
Thus far I've mostly been modeling them together.
To get from 2D plane to 3D shape I generally start with a 3x3x3 distortion cage, grab the middle four CPs of the cage and move them forward (from side view) and then start to adjust and fill in shapes and details. There is always grunt work to do... fill in 5 point patches... add additional spline rings around eyes, nose and mouth... etc.