This probably was already mentioned, but a single-player version of Shores of Hazeron with less terrible graphics.
I've always wanted a game with the full action/rpg and with a decent empire building / city building / resource gathering side, with interesting citizens doing interesting stuff meanwhile, or in the background.
Especially if it's in space. So you could build your stuff, manage ships or simply go out adventuring. Then come back and hang your new exotic alien heads on the walls and artifacts for the researchers to tinker.
I expected Spore to be something like this, but nope. Northstar was a old project by Kerberos (Sword of the Stars devs) that seemed to aim at that, but that one is buried. I expected Fallout 4 to be a bit close to this with the settlements, but I guess I expected too much.
Planet Explorers is definitely trying to go there, but they are having so much trouble with other stuff in development I just bet any sort of advanced interaction with settlers (outside story missions) won't make it.
There's a rather crappy game called Hinterlands that does something like that, but is really limited and 'procedurally match-based'.
There's also Kenshi and Starsector, but I first found these games like 7 years ago. I'm not really looking forward to them now.
Maybe what I'm aiming for is too complex when it comes to AI (would need rather smart NPCs to keep it interesting as a singleplayer game), so I guess a mix of EVE with Hazeron with more PVE and less reasons for people to annihilate each other.
Still..would want to see a single-player version of that. Couldn't be more niche.