I second the conjuration and dual-destruction recommendations. My current char is a pure mage, robes only,companionless on Master difficulty and it is important to remain nimble and use valorous discretion. Use alteration (it only levels up when cast in combat, so I've taken to having it ready to single cast at the first sign of danger, then switching to appropriate combat spells). Don't forget the illusion school - calm allows you to stroll by mobs that will one shot you, and frenzy will help your summons to soften up groups. Level illusion up to 50 and you can silent cast, which makes sneaking very powerful.
The beauty of dispensing with armour, smithing and combat skills is that your spell skills will rise faster as you level.