The guild was formed in 2e 230 with the approval of the PSJJJJ
The Mage's Guild grew quickly, but what kinds of magic would it have, the point of the mages guild was to offer magical items and potions for anyone able to pay. So there should be no qualms about adventurers learning spells.
I am unsure about the state of necromancy, the PSJJJJ disallowed it and for a while the Guild's purpose was fighting Mannimarco, also from the PSJJJJ. It would make sense that necromancy be illegal, at least in morrowind and summerset.
(i said illegal, I didnt say 'not in the game')
Mysticism in my opinion -must- return as in the time of the PSJJJJ all magic was considered mysticism due to their reverence for their dead and spirits of their ancestors, perhaps the development of the magical schools has only just begun?
I ask you all read up on what lore you know of and what you can find and share your opinions here.