But expansions are usually the complete opposite of the vanilla game, like Bloodmoon or Shivering Isles.
Expansions are also usually very similar to the vanilla game, like Tribunal or to a lesser extent Knights of the Nine (not an expansion, I know, but feels like one).
At any rate Bethesda are not going to do expansions. They've said this a few times before the game was released.
No, they'll be releasing Point Lookout (Fallout 3) sized dlc. Which is around half to 3/4 the content of an expansion.
To add to this they also looked interested in doing house dlc again and not horse armor again, thankfully.