I think Restoration is easily the most important second skill, as it helps you get the full value of your Illusion spells. The Necromage perk boosts your illusion spells against undead, and you will be grateful for that against Death Overlords. The spells to heal others to make your Rally spells more useful (I mean by making the NPCs you've rallied survive longer). And of course, until you have Master of the Mind, Illusion is helpless against Undead, so Restoration's Turn Undead spells are extremely valuable.
After that I think the choice is really down to roleplaying, and what you'd prefer to wear
After that I think the choice is really down to roleplaying, and what you'd prefer to wear

Using restoration is a good idea, but it levels so slowly. To get necromage you have to have 70. It's going to be much easier to get illusion to 100 than to get restoration to 70.
