These are good suggestions, but I'll add a few of my own.
In-between formables between planetary unification and the Solar Federation, like how you can form America, the Anglosphere, then Western Civilization, then the UNE as, say, an American remnant state. The UNE can have the Moon's cultures accepted as a treat.
Inner system versus outer system cold war? Yes please.
The battle of destiny between the furthest frontier and the original metropole, for the greatest prize of all: control of the Solar System.
Also please, some way for new cultures/existing cultures to show up in places they make sense over a game. Seeing half the solar system being Urbanite/Yankee/Russian/Han/Japanese/Lunar-American due to colonisation of empty regions when truly Martian cultures developed, in the backstory rather quickly is something... I don't know, depressing?

honestly a rework to diplomacy and peace deals would be really cool, and also some events for flavor like elections for democracies, terrorist attacks, etc. Then maybe a couple provinces could be split up like iceland and greenland, burgundy, and switzerland from austria (also california and nevada need their borders redrawn)

There is a suggestions chat on the discord you can use for suggestions like this. And yes, diplomacy is going to get reworked in the future!