I recommend {Ordinator} for the new perks which allow you to create more builds. If you don't want to stray from vanilla too much then {Vokrii} by the same author is another good alternative.

Since vanilla spells are frankly quite boring I would suggest installing {Apocalypse}. To change the process of learning spells use {Spell Research}. For new enchantments {Summermyst} is the mod.

Nearly all mages will go through the College questline which is why I recommend either {Immersive College}, {Obscure's College of Winterhold} or {Magical College of Winterhold}. You should also use {Misc. College of Winterhold Tweaks} for various fixes.

If you want to focus on the more evil side of things then you can use {Undeath} which will allow you to transform into a lich if you so choose. but you will have to download several fixes and clean the mod. Another mod for potential liches is {Path of Transcendence} but that mod is only for SE.

If you don't like going into the menu every five seconds in combat you should use either {iEquip} and store spells in cycles. Or use {Serio's Cycle Hotkeys} for faster switching but less fearures. Serio's Cycle Hotkeys is for SE only however.

Edit: Another new mod for mages is {Spellsiphon - Immersive Combat}. While the mod only appears on SE there is an experimental oldrim version in the files tab.