I don't think that adding more DLC's is pushing boundaries. I think there are still more story lines that Bethesda could pursue. I think there is potential for a few more yet. I would personally like to see something along the lines of civil war but with the people of skyrim at war with the Aldmeri Dominion / Thalmor. Obviously there would have to a story as to why this situation arises but I will leave that to the geniuses at Bethesda to decipher. From a stormcloak point of view this situation is plausible and almost feels inevitable at some stage. However from an imperial point of view it does not seem an obvious conclusion. My first thought was maybe the Aldmeri Dominion try to take control of skyrim and create a divide between the Altimer / High Elf's and the rest of the people of skyrim. Perhaps they begin to discriminate against and treat poorly all other races. Maybe this makes the empire change their opinion of the Thalmoor enough to participate in or at least not object to a rebellion against the Aldmeri Dominion. I was quite disappointed when I finished the civil war questline to find nothing is ever done about the Thalmor. Anyway that would be my dream expansion on the civil war questline for what it is worth. :)