Two brief question:
1) Is it impossible to base the DoT damage of these spells on player's level / ilvl?
2) Can you remove spells? Have you ever considered doing this? If you removed all of WHM's bar spells and gave WHM RDM's barspells along with a trait (say, WHM5) to make them aoe, and likewise for Protect/Shell, you would gain 25 slots of spells. As Jakuk points out, if you merge Gain-STR(etc) and Boost-counterparts, that's 32 slots of spells.
If you announced two months in advance that you were removing these, and made a single NPC available with all these spells (if a server-side conversion is just too difficult), that would give people plenty of time to update macros and buy the spells.
Something like--
Day 0:
Make announcement
Put WHM on RDM counterpart spells.
Add trait to whm.
Two months later
Remove WHM duplicates from the game