HP restored per potency scales with gear at almost the exact same rate as maximum HP. 100 potency of healing is worth roughly 4.0-4.5% of a healer's max HP and has been for the entire expansion.

As long as maximum HP increases with gear and mechanics deal integer damage, the percent HP dealt by a raidwide will decrease as gear improves and the effectiveness of a flat percentage healing spell will be equivalent to current healing spells. The only change would be that a healer will be more effective healing a better geared player and less effective healing a worse geared player.

Suppose a healing spell that restores 20% HP. On a player with 100k HP it'll restore 20k. On a player with 150k HP it'll restore 30k. If an attack deals 90k damage then restoring those two players to maximum HP will require 4.5 casts on the lower HP (less geared) player and 3 casts on the higher HP (more geared) player. The healer's own gear no longer impacts their effectiveness.

Edit: Making healing a percentage of the healer's max HP would work the same way as what we have now while still keeping the healer's gear relevant. That might make it easier to think about numbers, though difference between max HP on healers and max HP on melee or tanks will still complicate it.