I feel what would be a good fix for many absorb mechanics would be to make them only absorb 50% of incoming damage on a target. For example, if I have a Sacred Shield on myself for 40,000 and I get hit by an 60,000 Pyroblast!, the shield would absorb only 30,000 of the damage and I would take the remaining 30,000 damage to my health pool, leaving me with a 10,000 absorption shield still.
This would help smooth out damage taken for absorb mechanics without stepping on HoT healers' toes as much as it does now, and would allow absorbs to remain as mitigation mechanics.
I like this, and I also think it would make a proc heal like Earth Shield about as good as an absorb anyway; the only situation where it isn't is when the target is hit but doesn't actually take damage because they're overshielded, wasting the ES proc.
If damage was split between shields and health (any time shields are present), then shields won't snipe reactive heals and HOTs as much, resulting in not only better meter positioning for non-shield classes, but actually more stable player HP (because the shield will last longer rather than just forcing a bunch of overheal and then dropping).
Forcing an already existing effect to overheal is really just as bad as overhealing, but people look at meters and think "oh, the shield is doing lots of effective healing, it's those other healers who are wasting their heals on overheal". Damage splitting shields would fix that to a substantial extent.
The big disadvantage is that damage of 200% of your current health would kill you no matter how much shield you had, so shielding someone who is *already* at critically low HP would be less effective if someone else didn't also heal them at the same time. This would increase the importance of getting the shield up before the damage spike, because it could be less useful after.
If Blizzard wants shielding very low HP people to still save them as long as the shield is strong enough, they could modify your suggested rule slightly: if the total shield amount is less than current HP, the split is 50/50, but if the shield amount is greater than current HP, the split is proportional (if the shield is twice your current HP it takes 2/3 of the damage, 3 times your HP it takes 3/4, etc.) Then any hit less than your current HP+shield still wouldn't kill you. This could all be done with integer math so it wouldn't be computationally expensive.