That, and the physical time constraint of remodeling 800+ vehicles for Next Gen on top of completely rebuilding a brand new game from the ground up with a new physics model, career, and other features we've yet to see. I wouldn't count on this game being on par with Gran Turismo 6 (or even FM7) in terms of car count. My expectation is a better executed Forza Motorsport 5, with a larger focus on track vehicles, and a more modern volume of post-release content.
It was stated prior that tracks are more easy to make than ever, so I do expect a larger selection than FM7 on that front... but for cars? I'd be surprised if we get more than 300. Pleased, but surprised.
Thing is though, I don't think they'll have to rebuild cars and have another Motorsport 5-like situation with this next game. In fact, that's the one thing that fans wouldn't want: another Motorsport 5 where the car list has been cut down from a substantial amount. Even with a large car list already in the Xbox One era, there's still a large variety of road, racing and unique cars that are missing - particularly different and/or older model variants of specific cars.
They can still continue the huge amount of cars at the enhanced graphic quality as this new Forza is built on the same Forza-tech engine for many years, with its zenith so far being in both Motorsport 7 and Horizon 4. And take note: this is also a Windows 10 PC game, just like the two aforementioned Forza titles.
Surprisingly, even with no enhancements, Motorsport 7 is a backwards-compatible title (as it runs on 4K graphics too), so Turn Ten can polish on the assets from there and enhance it with updates to the graphics engine, besides adding plenty of new cars.
However Chris Esaki once mentioned in a Forza monthly that with ray-tracing there are some minor imperfections with some models with the ray-tracing feature so I'm sure they're also working very hard in correcting them as much as they are polishing older assets and adding new ones, and that they have adequate technologies to build cars at such a quick amount of time with a consistent quality.
Horizon 4 is also a Series X title (with optimized graphics and framerate), and still has a huge and diverse car list that will likely be expected to be ported over to the next Forza, along with many more from past titles.
I am welcome to a much larger focus on improving things like the gameplay features and mechanics, but I still want to keep the automotive diversity that also shapes the Motorsport aspect in Forza with cars of all kinds, no matter how serious or strange. Road, racing, off-road and unique cars (such as oddballs and bizarre cars) are what make up the series as a whole, regardless if it's Motorsport or Horizon.
I'm in it for experiencing all these cars with the more realistic physics system, let alone, this also opens up possibilities for cars that require complex physics systems too. But even then, with all these, licensing will always be the elephant in the room, being the only barrier that prevents cars from making the cut.
