first off; fully support the concept.
some ideas to kick around;
I am planning on doing a mod like this, so heres where I would be coming from with it. I intensely dislike the notion of building up some kind of metropol. I think part of the absolute splendor of blackreach is how perfectly balanced the areas are. I think that lore-wise, it was probably somewhat more extensive in ages past, but I would say keep burrowing, dont build up. I havent been down there in a while but I would "repair" broken or collapsed buildings and any construction done should be in true dwemer stylings; dig deeper! open up collapsed tunnels inside the existing buildings, add new tunnel openings etc etc. Current races simply dont have the know-how to build new dwemer structures, but cleaning up existing ones? well look at markarth, of course thats doable. Also I wouldnt necessarily branch into the connecting dwemer ruins. maybe make them static so once they are cleared they stay cleared but no sense biting off more than you can chew, blackreach all on its own is HUGE.
content wise, i like some of the ideas floating around. I think the dunmer city is the best one, and frankly skyrim has WAY too many this side or that side going on, morrowind had the great houses and they didnt get along but it wasnt this constant picking of sides and being reminded of it, it was just oh, youre hlaalu, go away i dont like you if you ran into the chance telvanni mage. The dunmer need a city since their homeland got scorched, also while i dont have anything against the argonians, im a telvanni at heart so no lizards ~_^
trying to replicate dunmer architecture could be interesting and there lore is actually in-game already to give it a head start. Brand-shei in Riften! the last known surviving member of house telvanni! also im sure ive seen some hlaalu's running around and we could probably dig up a few other houses from the dispossessed dunmer. Giant crab-shells are probly hard to come by in skyrim, but perhaps some clever redoran survivor could do something with the chaurus chitin that im sure will be in abundance as we merrily encourage the falmer to go elsewhere.
I would love to see a Baladas Demnevanni character to start lookin ginto the animunculi, perhaps Calcelmo could be convinced to move into blackreach and get his nephew to continue his research into controlling them. I wouldnt want a "summon centurion sphere" spell, thats boring and easy, I would love a control rod for several different classes of animunculi and you have to keep using it to keep it following you. makes it fun but tedious if youre in a hurry, so not game-breaking.
I think rebuilding and researching should be the focus. Political quarrels already abound, an archeological mystery/town builder sounds a lot more appealing to me. The characters are already there in-game so its just a matter of finding someone to script it, a couple people to voice-act it and perhaps some modelers/texture artists for any new meshes. Ive really just touched briefly on my ideas ive been storing for what to do with blackreach, so be sure that if the OP doesnt end up pursuing this in favor of the psijic order, once ive gotten my own personal mods taken care of ill be putting my brain on the task of making blackreach a habitable and inhabited city.
cheers!
~Star Boi
--Star-Boi-- I really liked what you had to say. As I did see some good points in there, I still hold on to my belief on a few things. For one, you are right about how huge Blackreach is. It's enormous. However, what I have also noticed, is it still looks just like mere ruins, there is way too much bareness to Darkreach, not counting the occasional building here and there. [A bit too spread-out]. Traditionally, now this is very important, cities, or at least the larger ones are closer built together. And there were some specific pieces of Alfthand, Mzinchaleft, and Raldbthar that I really loved. --For instance, while digging deeper into Alfthand, {the first Centurion you have to fight, where it opens up into a decent-sized cavern, walls, and I believe it was once a theater or something--.
That is why I like the idea of including Alfthand, Mzinchaleft, and Raldbthar. However, your mentioning of the Dunmer houses really caught my interest, as Dunmer are my favorite race and culture. Ah, I miss morrowind. However, once again, Lore-wise, and reality wise {based off of the personality of the Thalmer and Aldmeri Dominion, it's safe to assume that if Blackreach became a full-city, they would get their pants in a bundle and try and either investigate, or initiate authority over it, just to make sure Talos Worship isn't being performed and to keep an eye on the city.} +It's a Lore-logic thing, I am very loyal to lore. XD+