One fix all healers need is more potency on their attacks.... But for other things...
I think fixing WHM doesn't really need all that many new additions...
Just make Sacred Sight (and by extension Glare 4) work like BRD's Hawk's Eye... That every time you cast a spell, you have a % chance to get to use a Glare 4, maybe even introduce the mechanic at a way earlier level by replacing Freecure.
Make Presence of Mind into a raidwide buff, and add an attack that spends Lilies (Can easily use Seraph Strike from PvP for that).
SGE, give them a raidwide buff that gives Direct Hit chance.
AST, make Lady of Crowns be an attack instead of essentially doing nothing and other cards could be made to always usable, because stuff like 10% more overhealing is so amazing.
For SCH, Seraphism probably should be replaced as it's completely out of place on SCH.