Welp, I actually had a pitch for a Human Empire plugin that I was holding until I actually figured out dfhack/lua. Your ideas are really close to what I had in mind though.
My idea for the central "idea" of the human race was that money should rule everything.
The most basic custom building for the humans would be the Caravan stop, which would allow you to sell basic luxuries (Gems, Bullion,Silk, Spices, Wines, dyes) , and purchase Guild writs, which would work like the trade permit item for the dwarves. Guild writs would allow you to build Guildhouses, which would be the backbone of the progression system. Guildhouses would handle the training humans to castes, and would allow the purchase of building and item schematics.
For example: The basic forge would only allow basic, shoddy armor and weapons to be made (Humans are not natural craftsmen like the dwarves). However once the blacksmith's guild is built, you can buy the schematics for the advanced forge (which would take a few other tools like grindstones, calipers, hammers, etc). The advanced forge would then be able to produce any advanced weapon or armor that you've purchased the schematics for. You could then have an even more expensive forge available for older forts.
The other way that I felt that a gold economy could affect the humans, is to have them start at peace with every not-entirely-monsterous civ at the start, and have the different races declare war once the player has amassed enough gold.
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Another Idea that I had was that since humans are really the only "farming" race in a lot of fiction, why not give the ability to do horticulture?
There would be different "tiers" of each human-only crops, of which only the basic will be available. When seeds are produced, there would be a small chance to produce a cluster of seeds of the next tier (If this is not available, a horticulture station that takes in seeds and has a chance to return an upgraded seed would do).
An example would be:
Wild corn would be extremely low value food, and could not be used to produce much of anything other than animal feed.
Domesticated corn would have a higher value than most other plants, would grow in larger clusters, and could be used to make various powders/extracts and corn whisky.
Improved corn would be yet more valuable and efficient, and could be used to make higher tier food items, or even industrial ethanol for other reactions.