Seems like an idea that would naturally come but I've done a forum search and scanned the "already suggested" thread and found nothing similar.
Basically, I think large, somewhat rare, randomized cities that you can't see the end of on the in-game horizon would be pretty awesome, about the size of a typical biome. Here's a slew of ideas for such a biome, reasons why I think it would be such a grand addition.
-Walls and turrets spawning surrounding the main part of the city, with main roads going through parts of the wall
-Various NPC businesses lining main roads, NPC residences elsewhere
-A large somewhat randomized castle in the center of town, perhaps with courtyards or a moat or other castle-ish things
-large crop fields surrounding the walls of the city with small farming houses dotting the countryside, with docile mobs in fences and wheat and pumpkin fields and such.
-Loads of NPCs, perhaps such as:
---Farmers, somewhat aggressive towards fully grown wheat and planting seeds on tilled soil
---Guards, wandering walls, roads and castles and become aggressive if the player attacks a city resident, and are aggressive to enemy mobs
---Varied merchants, such as armorsmiths, butchers, pawners, etc.
---Beggars, scrambling for gold and food if dropped
---Hunters, in surrounding woodlands, rampantly shooting arrows at mobs
---Royalty, serenely in the courts of the castle
---Fishermen, casting lines into local rivers and ponds
---Monks, tending to buildings of religious study and silently preaching to other NPCs
-Randomized flags or emblems unique to each city that spawns
-Inhabited mineshafts, like the abandoned ones found but maintained, well-lit and full of miners
-Chests in homes, businesses and castles, all with different kinds of stuff in them
-Some vacant houses, so the player can take up residence in a city if they so choose
A city like this would expand lots of Role-playing options to a player if they were interested, and in general would just be a pleasantly overwhelming thing to come across. It could be expanded into interesting events to run into as well surrounding a city, such as traveling armies and sieges happening every so often, bandit encampments in the hills nearby, small caravans coming and going on the roads intersecting the city, perhaps things as imaginative as Nether-cults building portals and attempting to conjure up nether mobs. It would create a large new aspect of the game to explore and manipulate, it almost depresses me that it doesn't already exist.
This sounds more like something you would want to build rather then be per-built for you. It would take away any challenge of the game; too many resources available for collecting or utilizing.
And FYI, there is another thread with pretty much the exact same title, but the person only wrote a sentence in the OP. Just saying it makes the first statement in the post look a little ignorant, even if you are justified because you added more detail.
I'm not completely sure about this but may be useful for adventure mode so that when you spawn you will be in a city biome.
It would, but adventure mode is meant to be used with pre-made maps, which is what this should be. Something constructed by players ahead of time to enjoy and explore, not something that is made for us. You can take an existing village and expand it to make a huge awesome city, but having it randomly generated seems kind of cheap. Also, something of that size would be really hard to generate and have it match existing terrain, the villages as they are now don't generate all that well.
A 'big village' biome would work better if it was pretty much a meadow, with a REALLY high chance of spawning villages, and maybe have new structures like watchtowers be able to spawn there too, to give it more fortified feel.
Having cities spawn would really mess with what Testificates have been known to believe so far. It's really up to Notch, but I don't think fully fledged inhabited cities would be in the spirit of Testificates.