Ruling out Black Marsh because Argonians are the predominate race is stupid. According to every bit of lore we had leading up to it Skyrim was almost completely Nords. Then we get there and half the population is elves, beast race, or other form of human. Bethesda will change the populace to better allow diversity if they set a game anywhere. Besides that Morrowind was pretty Dunmer heavy but that didn't stop it from being diverse enough to seem realistic that you could be any race. I didn't watch the video but it sounds like this guy is just using his own biased for the lands -he- wants/doesn't want to see.
Personally I'd most like to see Black Marsh so we could get an alien landscape feeling again like Morrowind had. However, I think the most likely province is going to be something under Aldmeri Dominion control. The way Skyrim was set up it's almost certain that Bethesda wants that conflict in the spotlight. Perhaps a combo of Valenwood and Elsweyr like others have stated.
This is true, but according to previous lore sources Skyrim was completely frozen and inhospitable to all but the hardiest of Nords. Turns out it's actually not all that frozen at all in the southern half and actually has a rather welcoming environment for other races, even Argonians and Khajiit who have no business being able to stand that kind of cold. Lore also told us that Cyrodiil was a jungle... well we all know what happened with that. Bethesda just has to retcon Black Marsh to be less unforgiving (and you know Bethesda has no problem with retconning) and you got a place where others can live relatively comfortably.
For the record though I HATE retcons period, but if they can retcon Cyrodiil and Skyrim then why the hell can't they do the same to Black Marsh?
I think Elder Scrolls IV: Alinor would be perfect. The Altmer love architecture, so the whole place would be beautiful. You could just spend hours walking around, looking at all the buildings!
Possible storylines are plentiful. The next Great War has begun. The Thalmor and another political party are battling for control of Alinor. Hammerfell has declared war and is launching attacks on Alinor's coast (allowing you to control a ship, wouldn't that be cool).
Actually, it'd be neat if the game allowed you to import your Skyrim save (like in DA:2) and some of your important decisions decided what could happen in the game (who won the civil war, did you stop the dragons, are vampires ruling the day, etc).
Just my thoughts.