For the record, this is where my headspace has been for my own WHM ideas in terms of a rotation:
Stone/Glare: Your filler DPS
Aero/Dia: Your DoT. Upgrades to Aero III during HW and becomes AoE with falloff (other healers get to apply their DoT as AoE as well). Dia becomes AoE.
Water/Assize: OGCD DPS on a 40 second cooldown; restores a little MP. Assize is moved up to ShB as an upgrade, but during HW, Water III can be cast again while on cooldown to additionally heal the party. This transfers over to Assize where the initial cast is DPS and MP only, and the heal can be activated while Assize is on cooldown.
Arcane Ward: WHM Ley Lines. Is permanent, and can be repositioned once per 15 seconds. Every 5 casts of any spell while standing within Arcane Ward grants you 1 use of Tempest. (Also gains Arcane Warp which lets you teleport back to your Arcane Ward a la Between the Lines)
Tempest: GCD DPS, a little stronger than Stone/Glare. AoE with falloff. Combos into two other spells.
Soul Drain: Weak GCD DPS button (half Stone/Glare potency); restores MP and makes your next spell instant. Combos from Tempest, increasing it to DPS neutral potency.
Holy: Your AoE button. Same potency as Stone/Glare with falloff (would do this for every healer to rework AoE buttons to be usable in single target) but costs more MP. Combos from Tempest, generates 1 lily when used this way. Is your main way of generating lilies instead of automatically, but could also have a Rhizomata-type action as well that grants a lily for emergencies.)
Afflatus Misery: DPS Nuke reward from lily usage, as is currently. Can store a second blood lily, but has a 20 second cooldown now to prevent buff window dumping. Adds flexibility to when you use it.
Germinate: Channel ability that nourishes the blood lily once per 4 seconds over 12 seconds. Used for downtime so you're not obligated to burn lily heals frivolously. Cannot be weaved for a single charge as you must wait 4 seconds for the first nourish.
Presence of Mind/Temperance: Making Temperance an upgrade of Presence of Mind at level 70; retains the PoM effect and additionally provides an 8% Determination buff to the party which increases DPS and healing. (Old Temperance still exists as "Efflorescence" at level 80. I think the wings would be cooler to always see as a part of your rotation rather than a mitigation button, but this can also be swapped back to normal if people prefer it for the mitigation. It's just a matter of animation.)
Fleeting Familiar: New spell gained in the 90s during 7.0. GCD action on a 90 second cooldown (With the mindset of moving away from the 2 minute burst window). Grants 5 stacks of a familiar (spell effect around you, not a pet) that hits the nearest enemy for you each time you cast a spell with a very low potency (like 30 or so). Once all 5 stacks are consumed, nourishes the blood lily.
Again, just me randomly scribbling ideas down when I'm bored.
In addition to this, I'd like to give WHM a new form of utility in Float, which makes the party ignore puddle and ground effects for 15 seconds, such as the water in P2S. Would give a different type of mobility utility to your party. Perhaps up the frequency of ground effects in fights, even if less prominent than P2S, which allows WHM to allow for perhaps more easy positioning for certain mechanics.
Also I'd wanna rework Plenary Indulgence into a party wide buff that increases max HP by 10% for 20 seconds, as an answer to WHM's need for more mitigation, as this is effectively a refillable barrier.