I think colonization of the solar system should be a defining feature of the game.
Earth would the Western Europe of this game and have much fewer provinces than in the other Paradox games. Military expansion on Earth would be hard due to alliances and the constant risk for nuclear retaliation. Land, air and sea forces would exist but wouldn't be too complex so that we could also have space forces without overcomplicating things. Space forces would become more and more important as tech increases with orbital bombardment and asteroid flinging WMD's eventually becoming features.
The most powerful WMD's could decolonize target provinces and make them impossible to colonize again without the right tech. The user would get extreme diplomacy malluses and perhaps internal dissent too. Potentially a whole world like Earth itself could be depopulated this way, but humanity could live on elsewhere.
Space colonization would like in EU and Vicky becomes the fastest way to grow in territory and acquire new resources, but would require advanced technology and investments beyond what most countries can afford. Colonization and development costs would go down with better tech. Each colonizable world would like Earth have a number of provinces but most would be very small. You should be able to play as a privat company and then you would have no territory to control on Earth but would be the controller of provinces you colonize.
I think EU4 colonial nations could be a good model for managing the colonies. You should be able to release them and if doing so have the option play as them. In the mid game, well developed colonies should demand more and more autonomy which could end with complete independence. If autonomy isn't granted then rebellions would be hard to avoid.
There should an EU4 Holy Roman Empire-like mechanic for the independent countries of each large world (Earth, Mars et. c.) and each collection of small worlds (The Belt, moons around a particular gas giant). This in combination with colonial independence movements would be a key mechanic for setting up the end game. The weaker the combined members of a HRE are compared to the rest of the solar system, the more willing they would be to unite into one nation through the HRE mechanics. The first united worlds would therefore be colonial worlds. When these have become powerful enough, even the nations of Earth would become inclined to unite in order for Earth to not become dominated by the other worlds.
The design goal is for late game to have a political situation like The Expanse or Eclipse Phase and perhaps have the possibility of ending with some epic wars for unifying the solar system.