I've had this dream game in my head for ages where the game goes from a first/third person RPG to a city builder, to a 4X strat type of game.
You're just a dude in a big overworld that's at the distant borders of like 2 or 3 kingdoms. It's a pretty unremarkable backwater. You can just play the game like a very open RPG (not a lot of quest lines, just an AI DM that makes the game world react to your actions.
Once you get enough money and influence you can start building your own town. Have you build a fantasy village with farmers, black smiths, stables, build roads, etc.
Let you take control like a city builder, or just let the town grow organically with minimal guidance (which creates a sub optimal build, but suits a character more interested in being an adventurer than civil planner).
Once you get an important enough town it starts having influence beyond its own boarders, and you can go to a higher level where you're negotiating with other city states and kingdoms about military, political, and economic accords.
Put in some kind of indirect multiplayer where changes you make in your world impact other players worlds and vice versa. Like having some method of "banishing" monsters from your world, which just poops them out into someone elses world. You could have some kind of world ending menace that gets hot-potato'd from player to player until eventually it gets chipped away at by enough people, or lands in the lap of someone who is a bad enough dude to kill it.
I have no idea how you'd actually make the game, but I've got the whole thing planned out in my head.