All those who have played TES prior to Skyrim (or even the new guys that have gone back) know that the magic system of Skyrim is fairly limited compared to it predecessors. That's not to say the implementation is bad; in fact the duel wielding feature is probably the biggest boost to magic's combat effectiveness sense spell making. That being said there are many concepts that I feel Bethesda could expand upon in a DLC targeted toward magic users.

Rebalance the Destruction Spells:
Skyrim is a cold place. Most of it's inhabitants have a resistance or immunity to frost, rendering that type of destruction magic fairly useless. It's secondary attributes of stamina drain and slow don't quite make up for it. Perhaps if a chance to freeze the opponent was included in every frost spell along with a dedicated freeze spell, it wouldn't be so useless compared to fire.

The fact that fire melts ice makes it ideal to use in Skyrim. It's versatile and cheap, making it the ideal element to use in day to day Skyrim skirmishes. I don't think fire should be weaken in this DLC, frost and lightning just need to be buffed.

Lightning also needs so modification, just not as much as frost. Perhaps lighning spells could have a small chance to call down a fatal (instant kill) light bolt from the heavens if used outdoors. The bolt could also completely drain the enemies magicka within a range of 100ft or so.

Allow Elemental Fusion:
Bethesda is a creative bunch, we know that from the content of this very series. So why isn't there an option to fuse together different spells? We can already duel wield and combine the same spell in two hands together, this would be the next logical step. Imagine a fusion of fire and frost spells, you could create a cloud of steam with burns and disorients the enemy. Or perhaps summon and invisibility spell. Who doesn't want a spectral minion?

Reintroduce Lost Spell Types:
Does anyone remember charm? Open lock? Chameleon? Those spells were useful and actually had a purpose. Also it shouldn't be that hard for Bethesda to bring back some common sense spells such as those. Levitation however, maybe not.

Allow Spells to Affect the Environment:
In the event that the "open lock" spell doesn't make a return (or even if it does) why not allow the player to blast the lock open with a fire ball? This applies to weapons as well; I doubt a wooden chest can withstand a couple shacks from a sledgehammer. Other suggestions come to mind; tripling lightning strength/range in water, boosting illusion spells at night/in caverns, increasing the strength of dadric summons near shrines, etc.

There are other concepts which I haven't mentioned, or thought of, that Bethesda should include. Spell making's return being one of them. So what are your ideas on Skyrim's next DLC? Should it be magic based? And if so what concepts would you like to see?

I agree with the Lost Spell Types and the rebalance!