you shouldnt of did the hole lot of main quest..i made mistake too..but started off fromj a save before i did so...im leaving that way to the end..or until i close all the gates manually..have done about 10 so far..think theres 6 left..
and to answer question you can do all guild quests,such as mage fighters and thiefs..you can also do dark brotherhood and arena quests..and not to forget all random quests through out world..and then the knights of nine missions..theres alot still to do
