Alchemy, destruction perks, certain equipment in dragonborn dlc.
What I'm less sure about are other things, like I believe there is a glitched illusion perk that increases fire damage. And I also heard that taking the necromage perk increases destruction damage.
So if anyone can give me other tips for increasing destruction, or confirming the above, that would be appreciated.
By the way I'm an Xbox user so no mods here, but feel free to add in dawnguard or dragonborn content. If you do though make sure to add spoiler tags, because I put this in the vanilla general discussion.