Yes, that would be cool. I used to have a spreadsheet to make a custom car (parameter-wise), and combined with custom sprites, you could send that to me and I would but them in the game.
But sadly I usually got sprites that weren't high-res enough to nicely blend in my game.
But perhaps I can take your suggestion: put a template car online with the spreadsheet and and let the game import those files so you can drive the car without having to send it to me. (Unless it's so good that I want it to include it in my game!)
It won't be soon, but I'll try to do it someday!