There's actually an easy way to make the pure/shield healer dichotomy work: make shields almost as strong as regens in terms of total hps, with the main downside being a lower duration of the shield (around 7 seconds) and a higher mp cost to account for the fact that shields are inherently stronger than regens in the case of hps parity. At the same time, make it so that most oGCD healing skills require GCD spells to work, so more skills like plenary indulgence, deploy and emergency, horoscope, the old version of celestial opposition that extended regens and so on (currently op oGCD skills should be reworked accordingly). A lazy way to do it is, for instance, to make CO and asylum only work if there's a regen effect on party members. In general, oGCD skills should be a way to modify and/or enhance GCD healing spells, not the main or only source of healing in an optimized scenario (also make it easier to weave oGCD for sch and whm maybe by reducing the GCD of their main damaging spell to 1.8 seconds).
And finally, make unavoidable raidwide damage frequent and high enough so that at max ilvl it's not a joke to heal through.
Ideally, savage and ultimate content should mandate the presence of a shield healer by not making it possible to survive certain parts of the encounter without their shields. This can be easily achieved by making aoe shields strong enough (my idea is something like succor healing for 200, shielding for 400 with 7 seconds duration so that you actually need to pay attention to when you should cast it) and aoe damage frequent enough. For leveling content, just give an oGCD skill with a 30 seconds cd to the shield healers at level 45 that converts any shield currently applied on nearby party members to hp and that's it.
If they succeed in doing this, balancing healers will be easy, since they'd only have to balance each pair separately.