The game I have in mind is about exploring the depths of a modern city only to find all sorts of strangenesses thereunder.

New York City

2) Places:

3) Strange creatures or plants:  suggest two

4) Weirdnesses:  Just one. anything from Ancient Magic to Superscience or anything in between

5)  Cultures among the denizens of the Underground:  two each

6) Powerful things:  Lost gods, Alien Invaders, rogue AIs?  Only one

7) Factions: two

8) Classes:  two each?