I like Shivering Isles the most(although I consider KotN to be a mini-expansion, and it is in second place for me). I love all three of the ones you mentioned, however, and I'm confident that Bethesda will make a great expansion for TES V, if they decide to make an expansion for TES V. I'll let Bethesda choose, although I would love to see Moonshadow, or, depending on when the game takes place, the Imperial City in its current condition(the city and the Isle to make up the entire expansion).
- Oops sorry, forgot KotN, oh well that's what "Other" is for right?
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Moonshadow would be amazing, and yes, an Imperial city expansion would be cool, maybe more detail and added size. not too sure if Bethesda would do that however, seeing as though last game was set there but still sounds good. I'm thinking abbaccean isles, or however it is spelt, the oblivion mod for it looks amazing and a tropical setting could do wonders for the games diversity i think...