A healers artifact that makes the 4 man shield cover 8 people (when at 160). When in a group of 4, instead of making the shield cover 8 people, it either does 75% more damage reduction (additive), or places a heal over time on with the shield.
Healing has already become the easiest role. What you are suggesting to to broken. Maybe if it turned healer shields while in healer role turn to an 8man mitigation shield it may be more balanced. But giving healers an 18s 8man shield is just to much
I think a more balanced compromise would be 80: small group shields now cover 6 players but damage mitigation is only ~33% 120: damage mitigation is increased up to ~66% 160: small group shields now cover 8 players 200: damage mitigation is increased up to 100%
Given Bubble > Riptide > Bubble is a thing, this sounds like a good idea. I'd say it's inevitable. Eventually there will be an artifact or ally that's tailor made for vanilla healer shields. The simplest thing to do would be to add more shielded targets. Make it a 6-man shield for balance purposes if you must, but really an 8-man standard healer shield can fly, too.