Ooh, yesh, all underground sounds nice.
If I may, I'd suggest a single large cavern around the center of said thriving underground. Assuming this is a large-scale map, it could function essentially as a multi-tiered underground city, with many offshoots around nearer the edges of the map.
Also, as a rule, we should allow surface building, but only on a very small scale; say, a partially concealed shack entrance to the "city", an oasis (with sufficient built-in flood defenses, i.e., an adminium bowl with all but the outer rim contained, if that poor description tells you anything).
The only possible exceptions to the small-scale rule might be creatures. Say, a giant sand worm.
The only point of contention will be sprites. If I were in charge, I'd say for that map, no sprites allowed, as they'd break the immersion. But, I don't see why they couldn't be allowed along the outer edges if we do decide to allow them.
The only problem I foresee with an all-underground build is tunnel intersection. It will happen, even on a huge map. Some people more protective of their long tunnels might not like having other tunnels cross them. How we go about managing this, I have no clue. I'd say, no long diagonal staircases or passages, and no twisting tunnels (except in mazes). Try to keep it straight, unless you have to avoid another chamber.
Miniman has final say, though. We'll see what happens.