Thalmor questline: Join the Thalmor and plot to take over the world. Kill Talos preachers/worshipper, dececrate shrines, make sure the war keeps raging, lots of espionage.

Destroy the Thieves Guild + Maven: Nobler characters are probably craving this. It would be more fleshed out than "Destroy the Dark Brotherhood" and would include espionage/finding evidence, and then arrest/interrigate/kill the members/Maven.

Guard questline: Sign up with the city guards (it would probably have to be in a select city) and fight crime and curruption. Perhaps the guards could finally get that bandit raid they've been itching for and take out a larger organization. The most suitable cities would be Markarth or Solitude(placing it in Windhelm would clash with Blood on the Ice). Potentially you could also destroy corruption in Markarth. Or perhaps it could stretch over the entire map and even include "Destroy the Thieves Guild" with you being promoted to some kind of marshall travelling around Skyrim and fighting for justice.

Bandit Guild questline: Join(or even better - FORM) a bandit clan. Attack travellers, wagon trains. Loot cities, kill guards, make alleys across Skyrim and become king of the underworld.

Necromancer questline: Join a band of necromancers, a possible road of activation could be from the College questline, though setting yourself up as a master of the dark arts could also grab their attention. It would include killing various people/exploring dungeons for magical artifacts, perhaps in order to raise some creature/evil doer, trap a god in mortal form, and/or perhaps opening the gates to Oblivion.

Dwemer questline: Finding out what happened to the Dwemers, or at least attempting to. Could be a complete dead end, or just a hint - I'm assuming it's being saved for a future game. Perhaps the same thing that happened to Arniel could happen to you.

A dream related questline: I also wish there were more quests activated by sleeping. Would love a nighmare quest or two. In fact there could be an entire questline mostly based on your dreams. Someone could be trying to contact you, telling you stories in visions, then darker forces could invade your brain and you'd have to fight them out. You'd meet up with whoever is trying to contact you in real life and perhaps defeat the vilains real life powersourse before going back into dreamland to defeat the monster. In fact this would have been a badass extended Dawnstar questline.

Even my thief character would probably kill/imprison Maven If he could.

The Thieves Guild should be a largely independent faction, not a single noble's plaything.