A thing to note with animating shields is that most upped shields are cheaper. Not necessarily a problem, but still important.
Mito'ing a weapon causes it to generate new *fresh copies* of a weapon - so in the weapon slot. It can be reasonably assumed that the same goes for a shield.
Also, having multiple shields can become very strong very quickly. Bone wall is indeed a big one to combine with damage reduction and/or a killing shield. But even simply stacking things like skull shield is very strong. It'd need testing, since as a creature it can still be cc'd, but I think this card is overpowered.
I'd definitely not give this ability to any element, keep it in earth.
Thinking about this card, I came up with something quite similar to it, namely turning any shield into a golem defender:
2
| 2
Target shield becomes a 0|X creature, where X is 3 | 4 times its cost. It has Gravity Pull.
of course, it would keep the shield effects, but exclusively for itself.