Well, from what I remember, DF biomes are a continuous region of stone layers, dirt layers, flora, and fauna that are kind of grouped together. Lots of that doesn’t apply to what you’re doing yet as every type of zombie appears pretty much everywhere, except for some of the special types that spawn near map landmarks (like fungal towers for fungal zombies, etc) unless you’re planning on implementing some major changes in map generation. I think this is what you were getting at earlier with area “personalities,” like an empty region with just highways and gas stations to smaller towns, to larger cities.
I really like the direction the game is going, and I can’t wait for the release tomorrow!