Well, the name of the school "mysticism" implies communion with "hidden things" and it was used as as catch-all for spells which didn't logically or neatly fit into the others schools. They did away with it in Elder Scrolls V: Skyrim. Ma'iq even comments on it as being impractical. Somehow, I doubt that Skyrim is a good place to look. Outside of the Shouts, Shalidor, Ahzidal, and the Dragon Cult in general, the land is not known for being rich in magical history. If the Arcane University was Harvard, The College of Winterhold is a small liberal arts college.

EDIT: I suppose the Forsworn and Nightingales have their own forms of magic, but that's mostly Daedric stuff. Solstheim is part of Morrowind in this time, so it isn't Skyrim anymore. I suppose there's a bit more magic to be found in the Volkihar vampires and the Dawnguard. Not sure what to make of the Dark Brotherhood's stuff. I guess that's from Sithis or something.