I think WHM could be significantly improved with just a few tweaks and additions:

Shell
30s cooldown.
Bestow a 10% damage resistance buff on target for 15s.
Additional effect: Magic damage received by target has a 20% chance of granting you a Lily.

Protect
60s cooldown.
Bestow a 5% damage resistant buff on all party members in range for 30s.
Additional effect: Physical damage received by targets has a 10% chance of granting them a confession stack.

(Shell/Protect would stack with each other)

Lilies are generated only by Medica and Medica II, or by hits received via Shell.
Lilies are only spent when using Cure, Cure II or Cure III.
1 Lily = Cast time of Cure/II/III reduced by 0.5s
2 Lilies = Cast time reduced by 1.5s
3 Lilies = Cast time reduced to 0.

This would give WHM some useful utility in Shell/Protect, and the other healers losing Protect would give WHM some edge over them. They would interact with their Lily mechanic, which in turn would be useful in granting mobility.
Mechanically, it’s not a major shift to the way they function, Lilies are still pretty much passive but with a greater effect, it’s just two additional cooldowns, and WHM doesn’t have enough buttons as it is if you ask me.

Then you only need to add in a couple new dps spells.