I like the idea of shield stacking, but it would require a fundamental difference in how healing is designed. Right now we have very scripted damage, often high enough to one-shot at low ilvl, and healers are expected to keep everyone topped off. Additionally, heals are incredibly powerful in this game, to the point that healers can take a full party from 1hp to full in about 5 seconds. Sylvain also correctly pointed out that preventing the damage is always better than healing it afterward (especially in this game where secondary effects of attacks are often negated when 0 damage is taken).

You would need to change it so that there are very few one-shot level damage mechanics, and replace them with lots of random low-to-mid damage hits across the party. Then you would need to alter healing (or alter HP values) so that heals are less powerful overall. This would would make shields and direct heals/HoTs more balanced and allow for stacking since both will achieve the same effect over time.

I'm not sure how well a change like that would go over with the strength of our other skills, though. It would mean healers casting more heals all the time, and may necessitate nerfing MP regeneration skills so they don't just spam their strongest stuff when crap hits the fan.