Skyrim features the Forsworn business in Markarth which leads to the player spending time in prison. In light of popular tv shows like Spartacus, something's clearly missing. Since there's already a crime system in TES, that should be expanded upon.
If a player commits a serious crime, or commits many crimes, he/she should inevitably be thrown in prison. This should fail the main quest line, since the player can't participate in a civil war while rotting in prison for a few years.
While in prison, the player would be stuck in a cell in the evening and thru the night. By day, the player should be sent to the mines or quarries for forced labor. If the player gets into fights with other prisoners, the guards can take notice. Then the Warden decides to sell the player into slavery and ships him/her off to a Lanista, to be trained as a gladiator. The player will train and serve, fighting to the death in arenas and coliseums, hoping to win freedom, or perhaps planning escape.
If the player escapes from prison, then he/she will be on the run as a fugitive and an outlaw. You wouldn't be able to freely visit towns without a disguise, but you should be able to converse and trade with bandits and necromancers.
What do you think?