Okay. Just hear me out.
What if we entice car enthusiasts into doing this ecological atrocity promising project across the Sahara. The millions of kilometers of roads for infrastructure could be made in an interesting shape (and the Sahara does have some mountains, so winding mountain roads!).
The more I think about it, the stupider it sounds, but hey, Forza Horizon has a basically impossible set of requirements in order to make it work. So why not take the most basic ones (interesting roads and vast infrastructure) and make them a reality?
Just make sure you get rid of speed limits.