For ES VI, I'd like to see a return to Cyrodil. Skyrim was a good setting, but everything blurred together. Cyrodil was a nice fantasy setting. It's what I expect in a medieval, Tolkien type setting. There were times in Oblivion where I'd just stop and stare at a lake, or tree line because it was so pretty. In Skyrim I always seemed to be on the move, and only stopped to appreciate the view on mountain tops.
There's a laundry list of things I'd like to see return from both games, and see improved upon. The combat and leveling from Skyrim, the create your own spells and item system from Oblivion. Older ideas like guilds, and newer ones like the marriage, adoption, and house building from Skyrim. Maybe add in improved farming, training of followers, more weather and seasonal changes.
As for new things, I'd like the inclusion of multiplayer. Don't get me wrong, ES is a single player game to me. However I'd like to be able to invite people into my home after I spend hours decorating it, or have someone tag along in a dungeon.
I'd also like to see the difficulty go up a little. Oblivion messed up by increasing armor on enemies as difficutly went up (i.e. Goblins appearing with glass mail). Skyrim just became too easy, even at max difficulty setting I was still one shotting things towards the end.