After a lengthy break I just bought the Skyrim DLC and remodded the game and got playing again. It plays better than ever now the game and community content has matured. I probably also agree that this engine has gone about as far as it can, let's hope for genuine progress here.
As for the next ES game, I would predict that Summerset Isle or Valenwood would be the next setting as you can't go wrong with elves. I think getting most of your game's lore from Khajiit or Argonians would be a bit offputting for some people. I'm not ruling out a return of the Dwemer!!!