I'm surprised that only few to none of you mentioned about strategy and management games. For me, combination between Cities Skylines and Civilization 6 would be a pretty good.
1. In Civilization 6, we are able to build our empire from Ancient era, which Cities Skylines don't have. Watching our empire evolving from very basic era to advanced one is extremely fun. For me, this reason alone would be enough on why this game should be combined.
2. In Cities Skylines, we are able to manage our city in a very fine way. Almost everything are simulated, like the citizens. Even small details will give huge impacts to the game. Civilization 6 lack this micromanagement aspects. Other than adding more challenges to the game, this feature would allow us to fine tune our empire at the smallest details possible.
3. Cities Skylines also lack in war mechanics like in Civilization 6 while Civilization 6 lacks in natural disasters like in Cities Skylines. In Cities Skylines, you are unable to attack another player like in Civilization 6. While in Civilization 6, the climate in the game seems to be constant not like in Cities Skylines. There are no volcanic eruption, tornado, etc. which would make the game more interesting. This combination would solve both of the problems mutually.
4. In term of game progression, I would go for real-time management like in Cities Skylines, instead of turn based like in Civilization 6. I'm pretty sure almost everyone will agree with me on this one. Real-time management give you little room to think, and you need to act fast. This is good for training my brain.