With an ever increasing hp of creatures in legendary quests, it's much harder to kill mobs the old fashioned way.
In order to compensate for it, we need to either increase passive Equipment Bonus Spell Power cap in items or we introduce more items that increase caster level of spells up to level 9.
For Spell Power, it's fair to say that elemental Spell Power should primarily get the boost as healing could get overpowered.