Basically I will describe my thoughts couple years ago,.
Back when elven/dwarven sites were disabled in adventurer non-travel mode I played with making all civs build towns (and castles) and thought that we can have the basic town structure but place it underground, enable tunnels instead of trees, etc etc. Then 40.01 came out, hillocks and sensible forest retreats made me forget (for the time being) about this. Now I once again recall that thoughts because of laggy dwarven fortresses and major human towns.
So, basically. The site generation rules can be defined in broad terms in raws, having the following information:
- name
- tile and color on map
- - and how it is changed based on population
- is it fast travelable or not
- is it used for special residents (like night creature types or new HFS)
- possible (trade depot, workshop room, stockpile, drinking mound, civics mound, living hillocks, farm plot, house, warehouse, different shops, market, inn, wal, keep, castle, outside wall, tower etc etc)
- - each can have POSITION, OCCUPATION, PROFESSION of residents
- the levels (for example, "above ground", "below ground-staircase", "below ground-sleeping quarters", "below ground-workshops"), their requirements (basically population, but maybe some other conditions, like CLAIMED_AFTER_SIEGE)
- the level structure.
So the human town will have listed all the names according to possible population, all the possible structure, and rules how to make them; like "place keep if population larger than 10, castle, shops and markets for more than 30, outside wall and sewers if more than 50; sewers are placed so and so; streets are placed so and so".
The modders will be able to add new buildings, or incorporate some player fortress tricks as wordgen game fortress tricks, and of course it would be easier to make some whacky civilizations based on this.
Well, I suppose, this will eventually be even without my suggestion, but I want to advocate more flexible approach for defining elements of sites, so they are limited by games physic and modder's fantasy only.