I think the Dark Brotherhood needs more variation in its radiant quests. The Thieves' Guild radiant quests give you a choice of pickpocketing, robberies, shilling, and so on. The Companions get you killing animals, rescuing hostages, getting into fistfights, and so on. Even the mages let you do different sorts of fetch quests. But the Dark Brotherhood does only one thing: murder people. And even there, the variation is negligible. You'd think after I'd killed the new lumberjack at the same mill a dozen times they might get suspicious when I show up.
There are all sorts of possibilities for Dark Brotherhood radiant quests which don't involve just murdering someone. For example:
* Intelligence gathering. You are sent to a random guardhouse to steal papers on guard schedules to permit one of your members to do a job. The catch? You can't kill any of the guards or they'll become suspicious, the job will be ruined, and you'll fail the mission.
* Intimidation. Just the threat of the Dark Brotherhood is enough to frighten people into compliance. You've been hired to wound someone, then escape. The catch is that you *must* be seen doing it in order for the victim to know the Dark Brotherhood is involved. If you kill the victim, the mission is failed. The trick is to do danage without killing -- which might be quite difficult at higher levels and lower difficulties, where a single blow is enough to kill 20x over.
* Rescue. One of the Dark Brotherhood's agents has been captured on a job and is facing execution. Your job is to break into prison and escape with the captive.
* Protect the target. Independent assassination must be discouraged. You've gotten word that an assassin plans to make a hit on someone. It's your job to prevent it! Shadow the target, wait until the assassin makes her or his move, then intercept and eliminate the assassin before the victim is killed.
* Hail Sithis! You are enjoined to desecrate a random altar, after which it will be blood-spattered for a week, during which time it will give a Blessing of Sithis instead of its usual blessing. After a week, it will be automatically cleaned and reconsecrated.