This is what true expansions are for, and sadly, I don't think we'll see any of those.

Hell, your "Huge 50 floor dungeon" is damn near an exact, albeit vague, description of Mournhold.



Given the relatively static nature of even The Elder Scrolls games, it's hard to justify continued play after the main threat (or sub-plot threats) are eliminated. At least for me, exploration, be it story-wise or world wise, is an important incentive to perpetuate a single character. Grinding for "Sweet loot dawg" doesn't really hold any allure once the primary conflict is over.

Maybe they could have a special arena or labyrinth and summon Mehrunes Dagon for people to fight, I think that would be very tough.