but I think you may be right it makes more sense as an advanced worldgen token. The first problem with this that I see is that the advanced world gen parameters are by default agnostic as to the contents of the RAWs.
which is why I said make DF read the raw when loading into the world gen screen,
I did a search for a few permutation of this topic and found no direct matches so here it is.
My suggestion is simply* to have DF read the entity RAW's when entering Advance World Generation and adding options in advance world gen that enable you to set exactly how many instances of each entity you want, entities could be listed by both [ENTITY:MOUNTAIN] and [CREATURE:DWARF], I see the default looking something like this.
Number of Civilisations : 40
Playable Civilisation Required : Yes
Minimum Number of Mountain Dwarfs : None
Maximum Number of Mountain Dwarfs : None
Minimum Number of Forest Elfs : None
Maximum Number of Forest Elfs : None
Minimum Number of Plains Humans : None
Maximum Number of Plains Humans : None
Minimum Number of Evil Goblins : None
Maximum Number of Evil Goblins : None
Minimum Number of Skulking Kobolds : None
Maximum Number of Skulking Kobolds : None
Minimum Number of Subterranean Amphibian Men : None
Maximum Number of Subterranean Amphibian Men : None
Minimum Number of Subterranean Reptile Men : None
Maximum Number of Subterranean Reptile Men : None
Minimum Number of Subterranean Serpent Men : None
Maximum Number of Subterranean Serpent Men : None
Minimum Number of Subterranean Bat Men : None
Maximum Number of Subterranean Bat Men : None
Minimum Number of Subterranean Ant Men : None
Maximum Number of Subterranean Ant Men : None
Minimum Number of Subterranean Olm Men : None
Maximum Number of Subterranean Olm Men : None
Minimum Number of Subterranean Cave Swallow Men : None
Maximum Number of Subterranean Cave Swallow Men : None
Minimum Number of Subterranean Fish Men : None
Maximum Number of Subterranean Fish Men : None
With this you could set it to generate what ever you wanted and because it is adding options by reading the RAWs it should work fine with mods as any modded entities would also get read and added accordingly.
Note *: I doubt it would be simple or it would probably already be in, I meant "simply" as in there is not much more too the idea then that.
I'm sure that you have seen that when you first select "Design new world with advanced parameters" you get the screen with all the ASCII icons and loading bars right?
I'm pretty sure that is when DF loading the RAWs up for use, during this process it could read the entity_default file, write the relevant options into the world gen file for use right then and there and once the player is done they can be removed until the RAWs are loaded up for world generation again.
I think it is probably best practice to make all the initialization files independent of the rest of the RAWs. Your suggestion would break this compartmentalization.
They would still be separate compartments, one would just read from the other as necessary.
Perhaps another file in addition to advance world gen parameters and/or an additional intermediate suboption that shows up after you set advanced world gen parameters that lets you tune things further prior to starting the world generation process? This intermediate additional parameter stage could include the details you describe as well as some other fine tuning details (maybe the ability to precisely place starting sites, which would be fun for people looking to exactly create settings like Middle-Earth or ASOIAF).
Your idea of dictating where sites start would fall into the world gen file as well, what I think you're asking for is for world gen to stop after geography but before history to enable you to choose these things then?
I think that this implementation would take more work and is something that would likely have to wait until the Myth Gen Map Rewrite as it requires breaking up the current world generation process into extra stages, I'm personally hoping this can get in after the Steam release but before the Myth Gen Map Rewrite which means not reworking the current system but working within it.
Have you seen the Perfect World utility?
It allows you to control the geography of your world in an easy and user friendly way and it merely uses options that already exist in the advanced world gen, here are a couple of examples of a more thoroughly designed world.
This is an old one from a previous version of DF done with a tileset, Australia
This is a new one from the current version of DF done without a tileset, Tasmania
The data for DF to build these world is stored in the world gen file, any option to say "Civilisations Start Here" would fall under the purview of the world gen file and not a separate file or sub option IMHO.
Edit: I feel this post comes off as hostile and I'm sorry for that, it seems to be an unfortunate habit of mine and I'm trying to work on it.
Also, The example of how I think it would look was when the RAWs are loaded, if you were to look at the World gen file outside of DF I imagine it would look something like this,
Number of Civilisations : 40
Playable Civilisation Required : Yes
Minimum Number of None None : None
Maximum Number of None None : None
After DF loads the RAWs it would read the RAWs entity_default file, DF would then fill in the Biome and Creature values of each entities generating new entries for each extra entity in the RAWs.
So DF loads the RAWs and reads the entity_default file, finds the first entity in the file check biome = Mountain, checks creature = Dwarf and fills in the blank spots changing the result to,
Number of Civilisations : 40
Playable Civilisation Required : Yes
Minimum Number of Mountain Dwarfs : None
Maximum Number of Mountain Dwarfs : None
DF then checks if there is another entry entity_default if it finds one it generates an new entry in the world gen file,
Number of Civilisations : 40
Playable Civilisation Required : Yes
Minimum Number of Mountain Dwarfs : None
Maximum Number of Mountain Dwarfs : None
Minimum Number of None None : None
Maximum Number of None None : None
Which it then fills in,
Number of Civilisations : 40
Playable Civilisation Required : Yes
Minimum Number of Mountain Dwarfs : None
Maximum Number of Mountain Dwarfs : None
Minimum Number of Forest Elfs : None
Maximum Number of Forest Elfs : None
rinse and repeat for all entries in all entity files, should make it compatible with all mods.