WHM currently has GCDs tied to resources. And if they didn't lose any potency they would have been just as good as an ogcd, even better since you would get the time needed to weave 2 additional oGCDs after using a lily. It is not like the current design is a complete failure, small adjustments need to be made to enable whms to play optimally.
What I mean by small adjustments is removing the dps loss from using a lily and maybe change benison so that it nourishes the gauge and gives one lily per cast. It is a water spell, after all, it's kinda thematic if you ask me. The same thing could be done for fluid Aura so that it stays relevant. Kinda like how Ninja's Mug Shot is relevant because it charges up the ninja Ninki Gauge despite it dealing almost no dps.
Revamp plenary. Make it a gcd spell that consumes a lily because most of my clipping comes from wanting to activate plenary before using afflatus rapture. It will be a gcd but it will have its own recast timer for balance purposes something like gnb's dot. Also, change what it does because to get the most value out of that spell right you will just end up overhealing. Maybe instead of having the 200/400/600/800 cure potency healed right now, they can be stored up for the second AoE. Or you know, it can just become a party-wide regen healing spell that comes with no dps loss kinda like Ast's Celestial Opposition.
Make Tetra a dps neutral water regen ogcd and make it charge up the lilies. You already have 700 neutral cure potency from Afflatus Solace.
Make Thin Air interact with your lily gauge. Instead of nullifying mp requirements for gcd heals make it so that it allows for the execution of lily spells without consuming a lily.
Last but not least, introduce the lily system at level 30, when you become a white mage. Ast and SCH both have their gauges before level 50. Also, remove the passive charging of the lily gauge. You already have 3 ogcds from which you get lilies. You need to start pressing your buttons to have access to your whole kit, just like sch and ast. At level 30 you get Presence of mind as well, so this would be a great way to make that ability interact with your lily gauge. It can be smth like a party-wide that is unlocked only if you have 3 lilies stored.
Now adjust all of the cure and damage potencies so that the class is balanced.
This is the kind of change I would like to see for whm. It still remains a gcd healer which is its identity. The changes I proposed are not that big in my opinion. They are only expanding on what is already there and making different abilities interact with one another so that the person playing the role gets the sense that the skills are not randomly put in the whm's kit.