Okay, I've played around with modding some more, so here's another version. This time I've also added small chance for slaves to be born from goblin/orcs + slaves, and now 'special' colorations of orcs and goblins also come with special trait. Trait is inheritable and its precise stats are generated randomly. Still not sure how game selects which version of the trait to inherit if father and mother have different ones, but I assume it just compares trait.Value? Assigned it just in case. There's 1/20 chance for a goblin or orc to have special trait (works only for 'generated' monsters, so goblins you get at the start and 'fathers' that impregnate surrogate slaves at burrows).
Also - apparently, you can just drop your mod folder to StreamingAssets folder and game will pick up everything from it. So to install the mod just throw 'z_inheritableColors' to the StreamingAssets, it should work and override everything else (I assume game loads stuff in lexicographical order, hence 'z_' prefix to make sure mod loads last). As before, values are eyeballed, as this is more of an example of what you can do than a playable mod
Things I've also tried to figure out but couldn't:
Do something with deployment cap on stockades (increase it, or make deployment cost float so you can have for example 0.5 cost)
Do anything with skills, I think they're hardcoded for now
Make non-inheritable traits
Add some sort of special enemies to the waves (technically possible, you can easily add new enemy type with different stats, but no idea how to change visuals)
Adding cs file with various methods so I don't need to copypaste them everywhere (I think you can't, only methods that were there at compile time can be called)
Add some sort of 'medical room' to heal units before you get sisters (I think just adding room should be possible, but idk about healing)