What is a video game that you'd love to play, but no one has developed yet?

You can do most of this in Cities Skylines - certainly pedestrian view. Not so much driving a car or a plane but there were mods that did some of that I think.

Cities 2 also has a pedestrian view and there is a photo mode which allows some flying but not a freeform driving/flying mode as you describe.

GTAbut you built the city

Murder is optional. But it would help test out your emergency response facilities!

Anno 1800 lets you go for a walk in your cities, and drive cars and tractors when you build them. I think it was one of the F-keys.

I would prefer a civilization builder with that kind of personality. Live civ 5 mixed with baulders gate 3, with loads of asthetic choices for my towns, other civilizations to go to war with. Etc. Maybe you could play from the perspective of God, play the majority of the time in a top down cilization style, but then also have the option to go down to your planet as an avatar or profit or whatever and lead in a more hands on fashion to solve different issues. Say your chosen people are losing a war, you could become a great warrior and come down to help for a couple years. Idk, I just really love the idea of a civilization builder that feels more personal and involved.