I personally try to pull of the whole evil mage/necromancer/Voldemort gimmick, and I quiet enjoy it.
The fact that there's only the College of Winterhold, which is basically a good mage guild kinda dissapoints me. Where's the opposing faction? The Thalmor could do, but I'd rather have a figure like Mankar Camoran lead against the College.

Overall, I'd like to see opposites for each guild.
Dark Brotherhood / Guardian Angels (?)
College of Winterhold - Mages Guild / Necromancer's Committe (?)
Fighters Guild / Silver Hand or Blackwood Company (?)
Thieves Guild / The City Guards (?)

That's just an idea though. It'd probably be a lot more work, but it would most likely add variety to the players experience. The fact that oblivion had those opposing guilds/groups, but the player was not able to join them was absurd. It's almost like they encourage playing the "good" side, excluding the Dark Brotherhood.

Oh well, I can dream! TESVI

Dark Brotherhood Vs. Imperial Legion/Solitude Guard.
College of Winterhold Vs. Renegade Mages, the Dragon Priests, and the People of Winterhold
Companions Vs. Silver Hand
Thieves Guild Vs. Aerin and Mjoll the Lioness.

Thieves Guild vs 2 people?