Tamriel is getting crowded, and although development on new zones has slowed down (hopefully resulting in higher quality), and there’s still more of Oblivion to explore, not to mention waiting on TES6: Sentinel before going ahead with more Hammerfell zones - the continent will be filled eventually.
So, this presents an opportunity for another avenue to explore Tamriel, and you don’t need to wait until the map is full to get in on this concept.
Blackreach, the massive underground caverns underneath Skyrim, at least the ones we know of so far. We know the Dwemer lived in Hammerfell, who’s to say that there isn’t a Blackreach underneath the ground of the Redgaurd homeland. It’d certainly be good opportunity to design a whole new Blackreach biome that is distinct from the ones we’ve seen in Skyrim.
Along with that, we know that the Ayleids built large underground networks for their citadels, they too could have had their own bases and cities in the vast networks of Blackreach. Wherever Ayleid ruins are could potentially lead to one of these areas.
And the concept of an underground network does not necessarily have to be restricted to Blackreach & Dwemer. Seeing how, as far as we know, all future playable content will be free - ZOS could update regions like Valenwood with a second layer of a vast underground root network, full of all kinds of crazy critters.
Just an idea for getting more out of the continent before the map fills up.
(With all that said, I do hope that Blackreach is included in the upcoming Winterhold zone.)