I would be interested in playing other people's quest if they had an interesting story to them. Like witcher 3. Given how creative a lot of fans are they could be fantastic. I don't think that would ever happen though as there'd be the question of vetting the content to make sure it is inoffensive and doesn't have mature content. A lot of user made stories would probably be very lore breaking.
I think I would make a "Real Housewives of Bloodshore" questline!