i was going through the mod ideas page for this site and i was noticing alot of guild/faction ideas im not sure if youd count this as a guild but how about bounty hunting u could stuff like question the NPCs for info on your latest mark bribe charecters to set your target up and that sort of stuff, basically i wont some missions as intresting as the dark brotherhood ones.

The dark brotherhood is basically a bounty hunters guild. Someone comes and tells them who they want caught, they send someone to get the person, the person who asked pays for that.

So it would actually be easier to add on to the dark brotherhood.