After doing "extensive" testing with a frost "mage" build (he has many other non mage skills, ie one-handed/mace, heavy armor, etc), I have come to the conclusion, and still submit that only a very small change is needed:
Fortify Destruction Damage Enchantments!
Well, "duh!" you say. But after really playing with this character for a while, it would be the only change needed to end all "underpowered" discussion. Granted, it could still be improved in other ways, such as variety of spells, et al.
But as a character that has the +25% damage perk, and 3 items that have +15% Destruction Damage enchants (the base effect with a grand soul gem and 50 something enchanting, but NO enchanting perks), that's +70% damage for every spell cast. Based on experience this makes spells like Ice Storm DEVASTATING. A couple of casts will make even Nords fall to the ground or get blown away. Now just imagine that with Maxed out perks (+50% from perks, +120% from Enchanting). It's unreal.
Fortunately, I'm on PC so I have access to mods that allow this to be done. Unfortunately, you HAVE to enchant your own gear as you won't find it out in the world, outside a few locations just to learn the effect. I don't always like to enchant...
If you wanted to really make it ridiculous, couple it with cost reduction enchants. I an almost see why Bethesda chose not to give damage boosts for spell enchantments. However, they SHOULD have.
But anyway, that's my thread. Let it devolve into another bickerfest!