My my my, we just loooove to hear ourselves don't we?
Are we talking about a sunken city with an enormous jellyfish hovering over it? The tentacles basically acting as a foil to three dimensional underwater movement as well as a randomizer for horizontal movement? Am I just jellyfish obsessed? (I managed to insert one in QBert 3D (weed is a hell of a drug...))I may make the 'hallways' more permanent in the 'mop'
How large are the tiles on your map? Are they large city blocks or small clusters of rooms? The size of the area defines whether you need to map everything or not.
I ran out of time, but also wanted to throw in some procedural fun for the GM in Irradiated Paradox, so I left the dark beneath the mushroom mountains blank and made a randomizer for finding exits from the hexes. I laid down a few fixed points and left the rest up to the GM's imagination and a roll of the dice. Since then there's been some strong condemnation of random generation here (which is making my tone a little defensive). I could argue, but telling people that they're wrong for not-enjoying something would be stupid. It's just confusing to me since a number of hits on the Tenfootpole list include systems for random generation including Gargantua, Ynn, Krrshall, Underworld Kingdom; hell, even Barrowmaze has a barrow mound generator!
It's possible that the answer to my question lies somewhere between Barrowmaze and Highfell, because the tomb generator is fun in the first, but the blank ruins-crawl surrounding the wizards' towers on the Drifting Dungeon felt lazy in the last. The tomb generator allows the DM to add extra barrow mounds for PC's who want to spend extra time on the surface crawling around the moor. It's a bonus that adds detail and possible side stories to the adventure. The ruins crawl on the other hand gave me a rich encounter and hazard generator but no tools to describe the contents of the hex (and just general ruins or trees icons on the map) which is just going to punish explorers with encounters instead of rewarding them with discoveries.
So, I would suggest either a detailed map with a randomizer for exits, or, if the area is too large to be accurately mapped, a randomizer for the content of the area (as well as encounters, hazards and treasures) and its exits, OR a hybrid system with a number of fixed, mapped points in your otherwise unmapped area, a roster of 4-10 geomorphs for when exploring one of the blocks/squares/hexes requires tactical detail and a detailed procedural generator for the rest. You might add to that a subsystem that allows for simple resolution of quick transit through the area for goal-oriented players disinterested in getting bogged down in map-clearing.
Much as I agree with people here and particularly our host that it's great to have a plug'n play module where everything's ready to go at the GM's fingertips, some things are just to big or impractical to cram into a regular product. I don't think that that means they should be left out altogether!