The aura idea is something myself and others have bounced around in the past on Alla. Basically limit it to buffs that function solely on duration and make the range itself something like 5 yalms. That should allow you to cover all melee on a single mob with your own Haste, Enspell, Phalanx, Spikes, and so on, while granting you the time you wouldn't need to spend casting on them to actually contribute damage. The cost being 1 MP per buff every tick should probably be an additional balancing factor. Naturally, once they move out of range, buffs go poof unless they were the individually cast versions.