What do you mean significant? Dawnguard added whole new mechanics (proper vampirism) to Skyrim's magic?
Vampirism is hardly an aspect of magic in the traditional TES sense.
EDIT: Not to mention that outside of Vampire Lord Form, there is almost nothing in the way of new spells. A few spells that do damage to undead but no offensive spells for Vamps outside of VLF. A few weak, weak summons that are really lame. They honestly should just bring back all the summons they left out of the game. In the vanilla game you only had the option to summon like 5-6 creatures.