There are improvements that are hard to implement and some not that hard. True galactic market is something not so hard to invent but could give huge impact in gameplay.
Why I call it TRUE galactic market? Because the market could sell only that players sell, it cant have any resources, ships, tech or whatever it is, on its own.
There could be 2 ways how to sell in the TRUE market:
Option A: Say a player have 20 Durantium that he wants to sell. The player could try to sell those 20 Durantium on his own, offering his own price in the global market, letting anyone who wishes to buy, or...
Option B: He could sell it to "market agents", the third party that acts similar to Mercenaries, that will buy anything at lower prices, as long as they have budget left. And then the market agents add those 20 Durantium to their stockpile and sell at its own, normal price.
True market I would say not that hard to implement, but having huge impact in the gameplay Now there is much larger initiative to build those trade routes, economic buildings n stuff and to be that filthy rich bastard who manipulates markets, buys and sells as he desires.
Lets say you want to weaken your neighbours by making them go to war to each other. Offer those war building resources to the market, exclude your friends from offers list and let those buildups take place! Fun! It also eases the players task to sell via diplomacy. Sometimes it takes quite effort to find somebody interested in mutually beneficial trade offer. Now you can skip that 1 vs 1 trade offer screen by simply offering via global (galactic) TRUE market.
That market would be even more true than the markets in offworld trading company game!