Damage shields in eso are WEIRD.. that is my official opinion..
so here is what i would do to make damage shields more realistically appliable...

Light armor passives now grant healing done per piece..
Damage shields no longer scale with health but instead with healing power (not healing received) and their resistance level is always the same at 30k phys and spell resist..
This makes it more applicable to and from mages. and because typical mages are squishy in light armor damage shields would be more of a magical extension to their health pool that offers them resistance until broken through (then they basically get owned)

in addition to these changes
All self cast only damage shield abilities cost magicka and the cost is a percentage of your max mag pool rather than a set value but their duration is indefinite or increased by a ton..
Ex. i cast a self cast shield before a battle and then i dont have to worry about recasting until the shield is broken (this is the same for damage shields you cast on others as well)
Damage shields cast on others for obvious reasons arent as effective but they still are a good choice for healers.. (low value cap like an 5 - 10k health shield)

If you cast a shield then unslot the skill the shield disappears much like sorc pets but you dont have to double bar the skill...

this post will likely be part of a bigger post concerning other areas of combat in the near future but feel free to comment anyway..