tl;dr: A Forza Horizon/Motorsport mix based in Germany (Nürburgring) with more race tracks and free roam and Autobahn would be the perfect racing game.
This is just a look into the future and what I would think would be fantastic:
I always thoght it would be nice if there was a real race track in a Horizon game what first happened with the Lego Update on Horizon 4. Unfortunately it was a Lego race track and I didnt like it very much. We now have the dirt Track and the road track in Horizon 5 and I think its fantastic but still there could be more race tracks. They are build so we can have many different tracks built thats also nice but it does not feel different enough.
Also with all the race cars in the game there is no real use of them on the open road cause you just cant handle the speed in the narrow streets of a city for example and because Horizon is focusing on Offroad I personally dont use real race cars often.
So what about having a mixture of Horizon and Motorsport franchise. My Ideal world would look like this:
Let’s make a Horizon Festival in Germany with the Nürburgring as Main Festival Area. It already is something like this in real life so this shouldnt be a problem to built ;). The Track is open for the public so this would be usable in free roam whenever you want. And when it comes to race tracks, you already have two: The Nordschleife and the Grand Prix Circuit. When players leave the festival it feels like a normal Horizon Experience, it wil be a small version of the most common landscapes of Germany like alps and coast and Autobahn of course. Maybe add Austria (Wörthersee - biggest Volkswagen Meeting of the world) and Switzerland?
That sounds like a normal Horizan game but here is the clue: Given that we all like to have something like a career mode, what about building your racing team from a garage you have at the Nürburgring. From that garage you can travel to all the racing tracks we know from Motorsport. This would then include everything we have in Motorsport like Rivals, Ranked Races…
Sorry if this is already a Work in Progress and this is a spoiler but in case it’s not feel free to do it like this 
PS: We need more Opel