Since WHM is all about raw heals, why not create a new powerful version of Protect called "Protect 2" for WHM only. Protect 2 transcends death meaning when a player dies and gets resurrected, WHM's Protect 2 still remains and does not come off (no recast needed). Also.. Protect 2 would act as a heal buff to receive extra heal percentages only from WHM. Like an extra 5-10% heal buff receive increased on players along with the usual magic/physical damage reduction (This part of damage reduction will remain the same as the original Protect).

The 5-10% heal buff receive on players will only work if heals comes from a WHM. Meaning it's not a 5-10% heal buff that can be used by AST/SCH. But I suppose if we're talking about synergy, why not let it be a 5% heal buff when heals is received from AST/SCH and 10% when heals received from WHM.

I say if keeping balance between the 3 classes of healers, let AST and SCH keep the old Protect and WHM have my proposed Protect 2.