Truth be told, at very high levels, pure Mages that max out Conjuration/Destruction with high levels of Illusion and Sneak can fit literally all niches of physical damage, magical damage, crowd control, and subterfuge.
-Dremora Summons replace your companions. They can act as a tank/distraction while dishing out high physical damage. Not to mention they're easier to place.
-Destruction spells with high friendly fire possibilities work well with this set up; you wont risk killing off your companions as Dremoras can be re-summoned easily. And you won't find them smacking back at you if they live (i.e. Aela shooting arrows at you if you deal enough Fire Storm damage at her)
-Illusion/Sneak to make sure you can just consistently spam Conjurations/Fireballs without even being noticed. Muffle is just amazing.
And Enchanting is your 'it'. With maxed out enchanting you can literally make two schools of magic require no mana to cast.
It comes down to what YOU as a player want to play as though =/. Skyrim is such an enormous game; find a play style you personally would enjoy.