You go to Vulkhel Guard/Davon's Watch/Daggerfall, meet a Cyrodiilic noble there, he tells you that his area of Cyrodiil has managed to stay out of the three realms war, thanks to their powerful militia in the area. Unfortunately, due to all the able personell being needed to protect the area against becoming dragged into the war, justice has pretty much collapsed, and they are now plagued by assassins. They need willing heroes from all across Tamriel to help fend them off as long as the borders aren't safe. You accept, get whisked off to the Anvil docks, where a couple of hooded figures approach you, and drag you into an alley. They tell you they know why you're there, and gives you the option to join them instead, but to do so, you first need to prove yourself. This has to be done by killing the same nobleman that gave you the mission. You then go to his mansion, and then have the choice of either following on with your original mission, and become an Enforcer, which will naturally make the DB hostile to you, or to say "screw this", kill him and by that get your initiation to the DB
Don't really see any good reason why this couldn't be implemented, and it would give those who play "good" characters a chance to also enjoy the DLC even it they don't want to be an evil badass.