It's day four and I don't really have a game yet.
This morning I banged together several of my old experiments to try and get a road network that would facilitate fast driving. I started with a relative neighborhood graph from a stellar-conquest strategy game I was attempting to make once (this is supposed to be a galaxy):
On top of this graph I layered corner fillets from a racetrack game I'd worked on at one point:
The end result is something that looks reminiscent of the roads in the Mad Max game:
I added terrain-dependent top speeds, so you can't accelerate as much as you like. If you drive off the road the car is forced to decelerate down to the off-road maximum speed.
This was starting to feel fun to drive on, at least for me. When I tried it out on my wife she had an allergic reaction to it, so I spent a couple hours trying to rework how the car behaves when you're trying to accelerate past top speed. It now tries to steer so it will eventually be driving in the direction you're pressing, so if you keep pressing Left you will eventually be driving straight left. There are still some nuances to work out, though. Even with that, the acceleration was hard for her to come to grips with, so I think I may pull in the top speeds some more and shrink things.
I've got to come up with some gameplay, so that's what I've begun work on next.