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

Sony tried this with ModNation Racers. It bombed.

Not the same kind of racing game, but Aero GPX is shaping up to be an excellent F-Zero successor that is going to have a fully customisable track creator.

Mod Nation Racers on PS4, you could do custom track. It was a really fun game and I liked the fact that you could defend yourself a little bit from items if you had enough boost and a good timing. Really had fun with that game.

There's a huge modding scene for Mario Kart Wii, and lots of custom tracks. There's also several track compilations. One of the most well-known is Wiimms Mario Kart Fun which currently has 449 custom tracks. It runs on a real Wii as well as in emulators. My wife and I used to play it a lot.

The tracks are made by various community members, but the compilation itself is made by Wiimm, the same guy that made the Wiimmfi service that lets you race people online even after Nintendo shut down the official servers.

And while we at it, I want a Sonic Maker game, which kinda like Super Mario Maker but for Sonic stages. Would be even cooler if the players can create a 3D Boost Sonic-styled stages too, in addition to the 2D Classic Sonic-styled stages.

Tuxkart

The physical Mario Kart Switch game with the WiFi controlled carts - except it's 100 % AR instead, and you can switch between 3rd person overhead AR view from your own position or 1st person view (VR-like) from the kart. You create your own tracks in the room you're in.