If there's anything people hate it's the, "go kill ten of x and save y," or the, "collect 20 of this plant," quests. I don't think I'm alone in that. But why do quests have to be that way when they can just act as the spring board for a greater adventure?
Especially with the guard v criminal coming with the justice system, I'd imagine there will be something similar with the dark brotherhood. These and other mechanisms give you tremendous opportunities to interact with other PC, especially if you make it so it's not as easy as just following a compass to your target.
The example I've always used is, what if say you steal from an npc, but randomly that npc works for someone else and you've inadvertently stolen from them in the process. Maybe it's a powerful politician or underworld figure, the point is first you're met by npc thugs demanding all your stuff because you stole from the wrong person. After you kill them you find a letter of symbol, etc. that can give you a jumping off point to figure out who it is. But at the same time, maybe they hire actual PC assassins or suddenly a large bounty appears on your head for the return of the item (if it's important, maybe it's not and it's just the principle of the thing. ie, know not to steal from me). Now you've got other PC looking for you and a price on your head, with the idea to either figure out who the random npc is behind it, or what the item is to stop it. Assuming you can get the price off your head, it may always be there baring some other "favor" quest, even if you've killed the npc behind it.
The point is by adding a few randomized quest points you get multiple people involved in an adventure that's not just about farming for xp or drops. And stories like that add depth to the game. And it's not something you can just look up a walk through for on the internet.
Especially if you don't have a compass pointing their way for either side. Instead make it so if you pass someone or enter a town and then someone on the guard or assassination quest comes looking with a poster, by talking to NPCs they'll say, "yes I saw them pass through here a day ago I think, they were headed towards the mountains." Or, "I think they may still be around."
That's just an example, but the larger point is by adding quests that aren't so linear you can bring many pc into their own mini-adventures, and it makes the life of the game so much more enjoyable, because even the simplest thing could be the start of something interesting.
The justice system, thieves guild, and dark brotherhood offer so many opportunities in themselves, but even beyond those groups you could create more. How many players would want to see quests like that?