Incredible idea.

I clearly could pay for the feature as a DLC. I always thought that civilization strategy management games really need more features in the economics side, at least more then; "Want more money --> Build a money district". The lack of economic mechanic really stances out for me in the late stage of a game, as your civilization grows physically, scientifically and technologically, there is not that complexification of your economic system of your country that you would think happens in real life. Your country is not a corporation, it houses corporations in its borders and thus needs to be a fertile ground to them. This is were the corporation system would makes sense in the game.

Your idees of unlocking access to certain types of corporation later in the game, progressively, in function of what was your gameplay centered on is a really lovely implementations of the way a corporation gameplay/mechanic could be added to the game. 

Moreover, the fact that the number of corporation that a player can have, vary with the quantity of territories that are controlled be his corporations in a similar way to how religions territories works, could permit smaller nations in physical size to still be relevant in the game be the economics control it could have on other countries and by still being able to have a descent industrial and economical output. Adding another level beyond biggest country = powerfullest country, as its not what we see in the real world, you still need to not have a falling industrial and economic system no matter the size of th country.

The fact that humankind has a very good politics and dogmes system also means that interesting's choices can be made by the players to, for instance, have an open/world wide market or a closed market, only accepting players via an accords inter-players or having customs and taxes or opened frontiers across adjoining countries.

Agreements between players add more collaborations and block players to "just go to war" with each disaccords. As the Humankind war system encourage already with the level of support that you get from the inhabitants of your country during war.

And I believe that if Humankind was a game with developed economics features it could really stand-out of the other civilization management games like civ6 that has more developed trading route features.