Well, they haven't really done anything in the Summerset Isles, Valenwood, Elsweyr, or Black Marsh, so I would expect the next game to be in one of those locations. They also haven't touched large portions of High Rock, Hammerfell, or Morrowind, so those might be tied in. Yes they have had games in those locations, but they didn't cover a large portion of them.
If they continue the series in chronological order like they have been, the next game will probably be set after Skyrim, but at the same time there is a lot of empty space in the previous eras they could play with.
The emphasis will probably be on a different race than the ones they've done so far. I mean Morrowind had the Dunmer, Oblivion had Imperials, Skyrim had Nords, and stuff like that. I would think the new emphasis would be for the Bosmer, Atmer, Khajiit, or Argonians since they are more or less the only ones that haven't had a game in their homeland.
I have a feeling the combat system is going to be fairly similar to the one in Skyrim since it works fairly decently. I'm almost certain being able to wield weapons and magic at the same time will stay at the very least.
New hardware systems mean better textures/models and potential for more space.
As for my personal wishes, I would like to see it in Elsweyr, but I think Black Marsh might be a little better since Skyrim was a little elf-y if that makes sense. It wasn't horrible, but the Dominion/Empire thing was almost a little too much in Skyrim and at times I began to think that if the Empire was replaced by the Dominion in the Civil War questline absolutely nothing would change. That, and just about every single Atmer was an asshole. The Summerset Isles might be a good place too since the Bosmer pretty much haven't had much of a role in the games so far. They were kinda just there for the most part.
I would like to see the consequences of questlines to be more like actual consequences. Like in Skyrim if you joined the College of Winterhold almost nothing happened outside of the College even though the citizens of Winterhold seemed to hate the place. Like if there was a simple go-and-fetch quest given by somebody who hates the college, you wouldn't be able to get that quest if you're a member of the college. Even something like general disdain would be better than what it currently is.
The marriage and adoption features are useful from time to time, I'd like to see those kept. Or the marriage feature at the very least because it's nice being able to have an NPC that you can sell stuff to.
Cities need to be way larger with a lot more NPCs. I don't even care if most of the NPCs are just randomly generated whenever you walk into a city, it's just that currently cities are boring. I swear Solitude has a population of 35 people for what's pretty much the largest city in the game. Also, make buildings multi-floored with those floors being able to be entered by the player. A fifty foot tall castle should have at least three floors, if not four.
Work on the AI a little more. Killing a bandit and having his buddies actually search the are would be nice. Certain guards patrolling in groups would make sneaking more challenging and less based on running up behind something and killing it.
Magic needs to be more balanced with weapons. Once you get up to level 50 or so in Skyrim magic starts to become significantly less useful for things other than transmuting ores. Having a spell level with it's respective skill would work great. A healing spell should heal more at level 90 than it does at level 15.
More sounds for better immersion. Running around in the woods of Skyrim without hearing anything feels pretty unnatural when I can do the same thing behind my house and hear plenty of things. Speaking of which, having every animal able to have an actual call would work wonders.
I'll probably think of other things, but that's it for now.