I love how magic is used in Skyrim, in fact, I prefer it over Oblivion (they better put spell crafting back into VI though!). The idea I have is a simple one: we have the three kinds of Destruction Magic. There’s frost, flames and sparks. It would be really cool, if you could unlock a perk in VI that allowed you to influence the weather with the frost and sparks spells. For example: by shooting sparks into the sky, you cause a thunderstorm that can damage enemies. The same goes for frost.
For the flames, I think it would be cool, that by sending a fire spell at a torch or campfire, that fire source would explode, damaging all enemies around it.
Also, the ability to have sneak attack damage with spells would be awesome.