This could actually be done with only small changes to the spec and a single change to the class overall.
First, move AP->SP to a baseline ability.
If holy heals end up too weak, buff Holy Guidance.
Make Divine Intellect a AP->int talent.
Buff Illumination back to 100% to compensate for the loss of regen on gear.
Currently holy is the only class/spec that uses spell power plate. This goes against the push to unify gear, to make it wanted for multiple specs and classes. Clothies all want SP cloth. The two classes that use leather also use AP/crit. Balance and resto can share gear, at least partially. Mail classes either use AP/crit or they are resto and elemental and can share partially. Plate classes all use AP/crit or similar tanking stats.
When holy gear drops, only holy wants it. When ret/prot gear drops, holy doesn't want it. No other spec has such poor sharing with others. Even the second worst, druids and shamans who would want much different amounts of hit, are still not so wasteful. They can make up hit elsewhere or for the healers, have a slight stat loss, but not a near-total waste as with holy.
Switching holy to strength gear benefits everyone. Holy can draw from a larger gear pool. Other plate classes don't have to groan when they see holy plate drop. Blizzard doesn't have to design as much armor, saving them time after the initial investment of converting holy.
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