Dark Brotherhood- I would like to see my assassin's guild expand and grow. Its not enough having 2 new initiates in the guild hall. I want this to feel like the "new" Sanctuary that i dreamed of rebuilding and maybe rebulding the sanctuary near Falkreath. Being able to assign my own speakers including Nazir and Bobette and a new addition of assassination contracts would be really cool. Also, I have no doubt people in dawnstar are getting suspicious of our little hall so close to their town. Maybe a new questline to "keep people in check" or maybe competing against the Morag Tong. They did have to leave morrowind to survive so maybe they regroup in Skyrim. Having to quest for better assassin's armor is a must.
Thieve's guild- Im not really sure what more I could ask of the thieves guild because it was done so right! I do recall attempting to take out the Summerset Shadows so maybe having to compete against them would be cool especially since they murder their victims and rob them blind which gives thieves a bad name. Hunting them down, stalking their agents in town, having to put stolen items in their homes or put stolen items on their person and watching them get executed by the guards I think would be thrilling!
College of WInterhold- I think questing for items of power would be sufficient. Mages do desire knowledge and power above all else I'm sure. Building another college for yourself to me would be really cool. Having to keep relations with the outside order in check, maybe mages could get involved in day-to-day problems of the world such as clearing out bandits and building up funds for the college. Then these funds could maybe be used to hire expedition teams including you to check out dangerous dungeons and find these items of power.
Companions guild- Perhaps having to deal with a rival werewolf or vampire faction since the dlc is about vampires. Or maybe gathering up artifacts of Hircine or even participating in Werewolf arenas in the name of Hircine. Or perhaps the fighter's guild makes its way to Skyrim and opposes the companions in some brutal Guild war.
I have to admit it wasn't easy coming up with this stuff. I can probably see why Bethesda may be having a difficult time with the guild quests. Anyone have any ideas for after-guild quests?

