Where can I find mage related questlines?

In the Dragonborn DLC, the Tel Mithryn quests are mostly mage-related. Lore-wise, this is because Neloth, which is related to and gives most of the quests, is a wizard lord from House Telvanni, one of the Great Houses of Morrowind. House Telvanni is comprised mainly of mages.

• Azra's Staffs: Retrieve a staff made by Azra Nightwielder for Master Neloth.
• Briarheart Necropsy: Examine a Briarheart of the Forsworn for Neloth.
• Experimental Subject: Help Neloth by being a test subject in his experiment.
• From the Ashes: Help Talvas Fathryon by killing an ash guardian.
• Healing a House: Help Elynea Mothren repair the tower of Tel Mithryn.
• Heart Stones: Find a heart stone for Neloth.
• Lost Knowledge: Retrieve the Black Book for Neloth.
• A New Debt: Help Drovas Relvi with his debt to Mogrul.
• Old Friends: Find the source of attacks on Tel Mithryn.
• Reluctant Steward: Find a new steward for Neloth in Raven Rock.
• Telvanni Research: Extract a sample from an ash spawn for Neloth's research.
• Wind and Sand: Retrieve a copy of Wind and Sand for Neloth.

^{Source: UESP wiki, "Dragonborn quests" article}

I completed the Winterhold College questline but after a while when I came back, speaking to Tolfdir (if I remember correctly) reveals more quests with some magical rifts opening up in the world due to the earlier events with the Eye and so on.

I haven't continued on this path myself but go back and talk with the people at the college after some time and you might find more quests after you've become arch mage.

How about the Forbidden Legend quest about re-uniting arch mage Gauldur's amulet? You can also test your mettle against Shalidor's Maze in Labrynthian - it's not a marked quest, but in lore terms was how arch mages used to be selected. There are also the ritual quests for unlocking the most powerful spells from each school, and Arniel's Endeavour (which comes in several parts).

Go to all the cities and talk to the Court Wizards/Mages. They can give you quests.

Also try out the quests for the Daedric Princes. Maybe you can fit some of them in your mage or magic related quests.

(And if you really like mages and magic, try out this mod: Midas Magic ;) )