I've reached level 70, and I want to be able to do stuff for many more levels to come.

Is there good end game stuff? I already know about dragon priest masks and the ritual quests you get at the college of winterhold when you reach level 90 in that skill.

It's not much, but from what I remember there's also the ebony warrior at lv 80, and then there's Legendary Dragons if you have the Dawnguard, which have no related quests afaik, but there's that achievement for kicking one's butt.