There is also the dragonborn main quest. I too crave long quests so I am currently collecting all the dragon shouts (except for the third word of slow time thanks to Hag`s end glich) and all the daedric artifacts. Hope that helps!
Agreed with Thieves Guild. Try to collect all dragon priest masks, all Daedric artifacts... A Niggt to Remember is quite long if you do not persuade anyone. Gauldur quest is pretty long
Thieves Guild. And it wouldve been longer if you cant change the jobs with its random locations. (Seriously, you can get stuck at whiterun for a dozen jobs more even if you already finished the specialty job on it, if you cant change jobs.)