There is no easy fix to this. Remember if you make the bosses do ranged aoe (i.e. just affect players far away from the boss) the ranged classes will just move in and fight him near the melee. The range do the same amount of damage close by the boss as far away, so it's no prob for them.
If the aoe is close by, the melee lose a massive amount of dps.
One solution here is to have all AOE with massive range ... so it affects both range and melee dps, to make it fair.
so, you want all fight become tank and spank, with no mechanics, no necessity of positioning and so on?