I have completed Dawnguard and the College of winterhold questline and need ideas on what to do next, I don′t feel like doing MQ since it doesn′t fit my character.

Do you have any of the DLC? Exploring Solstheim could be fun for a mage. Same with slaughtering vampires who have a distinctive hatred for fire.

Dark Brotherhood for destruction/conjuration magic or Thieves Guild for illusion magic.

Or you can just piss around.

If you are on PC then try becoming a Lich.