If galactic civilizations remove planet building I hope that is a pregame option I will never pick.
I think that is a bad idea.
The tall vs. Wide debate sounds cool until you look at the real modern world. I spent a couple of years on this. https://forums.galciv3.com/494320/there-is-a-tall-vs-wide-civilizations-debate-on-the-forums-here-i-compare-a
As far as I can conclude on the average tall can't compete with wide. Only the strongest tall can compete with the weakest wide. Average not tall beats wide in only one area. Unless you count GDP per capita. This means population. To my opinion this doesn't work because even a country with more money that is less than per person still will have more resources.
One of things I think would be cool. If after the whole galaxy is colonized if there were some way to get lost treasures other than colonization events.
I think during the last tech age. All random events, anomalies, crisis, and up decisions should switch to endgame stuff this would give you incentive to play the game longer.
I think the reason you will never see the black market is it is basically the market. If you want the market pick a different government. I know you dont like the fact it is part of the government weights.
I do think that something like the other black market option where players put there surplus goods for whatever price they want is a viable option.