Trade corporations wars and Grey Market system

That could work. At present, the only reason not to sink more Dust into a trade company is because you need that Dust for something else. But what if, as prosperity of the trade company grew, its ambition and power increased as well? You would have to decide if giving the company that extra freighter was worth the risk of the company deciding it didn't appreciate your taxes and hiring pirates to raid your outlying systems, or causing lowered approval in Pacifist planets...

Must admit, it perfectly fits idea of the game (about population moods) and could add colors to mid-to-late experience. At very least, it could be a bunch of Events