I think it would be extremely interesting to see a cold war+ geopolitical simulator. Make it focused on diplomacy and power dynamics, with dynamic treaties and organizations that can be made between countries. Early 1950s would make a good start date as the stage had mostly been set, and it could last until exactly 2000, letting modders deal with the explosive 21st century. The effective end would however be that of the cold war. Either one power wins, or neither.
The game could have some Vicky 2-esque economics focused on the rise of oil nations and eventually the growth of information industries and such. There's a lot you could do here, but I don't think it should be the main focus.
When it comes to internal politics, you couldn't do this without a proper political party mechanic, where each party/faction has a different ideology, economic practise, and a global alliagance.
Diplomacy would be the cornerstone of the game, with dynamic treaties which member countries can negotiate over time. Instead of a "form alliance" and accept option, you would instead open a menu with various deals and concessions. Everything would cost something so it isn't profitable to spam everything with everything, and your ruling party will become unpopular if you seem too weak to your enemies. Negotiation would also take physical time to negotiate, and can break entirely. Over time various treaties would stack up on another. Larger alliances/treaty organizations would consist of a variety of treaties bundled into one faction. Of course the largest of this is the United Nations, where a lot of interesting diplomacy could happen.
You'd need some good espionage mechanic that allows you to find out about both treaties and wars before they happen. In addition you could have counterespionage, tech stealing, and covert/open support of aligned parties. Military coups could stand as a very lucrative but dangerous move to push the global balance of power in your favour.
Finally when it comes to wars you'd have to go away from the binary at-war/not-at-war mechanic of other Paradox games. You could do something similar to HoI4's volunteers, but not the scripted limits to how much you could send. Instead sending your army would need a lot of political pressure and could be met by heavy opposition back at home. Armies would also have more political/diplomatic effects based on where you place them so you can't just move it all to warzones. In addition there's the danger of escalation, especially in the case of a certain type of missile.
So how would you win the game? I could imagine an end-screen which shows both how succesful your global alignment where. But in addition there'd be a general "state of the world" stat that rewards you for bettering the world, working for human rights and preserving global peace. And only then on a second page, you'd have the stats for your own nation.