Take note most of the areas involved in that quest were Vanilla Dungeons. I don't think they added any more entrances. Although, I personally want a DLC Dwemer Excavation via the collapsed dwemer tunnel in Tovalds Cave.
Yeah, that tunnel offers a good entrance to a Dwemer "Lost World". I was thinking the same thing about the Pass that leads to Morrowind. Maybe a visit to Mournhold is in order?
When I first entered blackreach I crapped my pants. I was thinking huge underground landmass with a massive inhabited underground city. I was so disappointed when I found out that it was significantly smaller then I first had assumed

So please Bethesa. Give us your take on a 'Tamriel Underdark' and blow d&d out of the water. (I like d&d though

)
Haha yeah I felt the same way. Big, deep, dark underworld and all I got was some Crimson Nirnroot and a dragon.
Really more than anything I want an expansion similar to Old World Blues in New Vegas. An independent landscape possibly dealing with an outside faction hell-bent on taking over Tamriel. Throw in some warp-able player housing (through a spell, power or item) that you can upgrade.