If damage dealing is the main concern, a simple buff may work. Add a trait that gives WHM a %dmg buff to WHM for each HoT she casts on her party, 10% for medica II, 15% for regen. The buff would only apply on the first respective HoT for each party member, so it doesn't result in ridiculous party compositions. Granted, I'm throwing out sample numbers and I'm in favor of much tweaking.
I expect it would do the following:
-re-establish WHM as the plain but powerful healer
-not increase the complexity of the job, unless players are trying to maximize dps (balancing mp, emnity, and timing)
-medica II dmg buff would scale heavily with full parties, which I believe was the main grievance of this thread
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#makeWHMsgreatagain