Destruction just needs scaling period. Magic needs scaling. For each level up in a magic skill there should be a small increase in the magnitude of the spell, about 0.5% per skill level. So Conjuration spells would last longer, Destruction spells would do more damage, Restoration Spells would heal more/block more damage, Alteration spells would last longer/block more damage, and Illusion spells would last longer/affect higher level enemies.
I put together another spreadsheet with things like destruction damage scaling, destruction damage perks, fortify damage enchantments, and tweaked the values around to make the destruction damage more or less match melee/archery damage (flames matches 2H, firebolt matches heavy archery, fireball is slightly weaker than firebolt but AOE.) I ended up with 0.5% bonus damage per skill level, plus a ranked damage perk for 20/20/30/30/50 (total +150% damage). The base damage for flames is still 8, but firebolt was reduced to 6, and fireball to 5. With all the damage boosts though, your dps curve would look like this:
http://i264.photobucket.com/albums/ii167/bl3count/rebalanceddamage.jpg
I also reduced the max fortify damage enchantment strength from 40%x4 to 15%x4. The same smithing improvement is used, but the numbers shown assume you're only getting +10% smithing per piece of enchanted gear, rather than +25%. Fortify smithing and enchanting potions are also taken out (I'm playing right now with a mod that makes those changes to enchanting and alchemy, and difficulty stays pretty good even with full crafting synergy.)
This tries to stick with the way spells are ranked in the vanilla game, but what might be good would be to swap flames and firebolt - you'd start out with the ranged, lower damage spell, then get the higher damage, short range flame spell at a higher skill level. Then for fireball, increase its damage to make it really destructive, but give it a high magicka cost. Magicka costs for flames and firebolt would be such that you could continuously cast them without running out of magicka, but fireball would deplete you quickly.