I agree with this entirely. The Black Marsh and Elsweyr are separated by a thin stretch of Cyrodil where Leyawin is located. If you combine the area of Elsweyr, the southern point of Cyrodil and the Black Marsh the map size would equal that of Cyrodil in whole so it is do-able. The possibilities would be endless with this, being able to visit the whole of Black Marsh and Elsweyr would give ES6 the most diverse map in and ES game yet. You would think that the Thalmor won't leave the Black Marsh untouched and the Argonians must try to resist invasion by taking Leyawin and holding strong. There would be Argonian factions joining together as well as pockets of Kahijt who don't agree with the surrender of their home land. Would the swamps of the black Marsh prove too much for the Thalmor giving the Argonians that extra chance. So one side of the map a desert land, one side swamps and forest with a nice slice of Cyrodil wedged in the middle. Who would you fight for??

The next game would have to be completely different from the pine filled cold lands of Skyrim and what better place to achieve that then by placing it in the Black Marsh. Damp, dark and misty swamps filled with creatures never before seen. Not too much is known of the Argonians and their native land, maybe something awaits the Thalmor there that they aren't quite expecting.Time to create a new race Bethesda!

Yes, I know of them, a marvel of construction! If I remember correctly, one of the fan interpretations of Nova Orsinium, I think it's called Orsinium Novum, the new Iron City as being mostly underground. I could easily see something like that in Hammerfell, and underground cities, perhaps built in natural caverns or around underground freshwater lakes, could really be useful in High Rock. Shornhelm, a mountain city built in an extremely inhospitable part of High Rock, could very well have a significant part of the city built underground. Towns in the West Reach could also make use of already existent and natural defenses, like the Direnni strongholds that dot the landscape, or in such natural caverns, IceFireWarden's Foaming Betrayed make use of a large Ayleid ruin as their "capital".

Here's an idea:

Say we go to Hammerfell in ES6.

There's some rourken dwemer stuff there.

How about a quest whereby your PC goes into a small rourken ruin, learns how to activate, deactivate and otherwise control the automatons there, and uses that small ruin - automatically defended - as his home base?

The question is how the PC learns to control the automatons and their activities. It could be quite an involved and interesting quest.

That would be .... so damn epic. Which automatically means that'll never happen without mods xD

Btw, are rorken's automatons different from others? Or is the golden/brass/bronze theme ever present with all dwemer's automatons?

That could work. The Rourken would probably play quite a major role in a Hammerfell game. The Volenfell Orcs and the Rourken Dwemer have some connection to each other, perhaps the Orsimer of Nova Orsinium uncovered some ancient birthright. They would probably hint and the ol' Dwarf-Orc theory at the very least. Perhaps Nova Orsinium is not a City of Iron, but a City of Brass? A re-purposed Dwemer ruin or something along those lines?

But for an undercity, I think the West Reach could work very well. As you mentioned previously, Highrock is no stranger to conflict, and none know that better than the Westreach Bretons, the Bjoulsae Horse-People, the Dunlish Nords and the Reachmen (damn, Highrock has a lot of cultures). Forsworn had made their blood-soaked camps in caves before, from frozen peaks, to crumbling forts, to Madanach's Druadach redoubt. Perhaps people who could not find shelter behind the walls of Evermor turned to the underground, building settlements in caverns and abandoned ruins? Could be visually interesting.

Enderal's Undercity of Dunewill is a good example of what I'm talking about, a large, functioning underground city, built within a freshwater cavern:

https://www.youtube.com/watch?v=GL8x66tKVmg

https://www.youtube.com/watch?v=ZtYQSxOjcZw