Some ideas I have for this game is a cinematic replay option, where you can add camera keyframes to paused or moving parts of gameplay both in free-play and racing, and you could make replay files stored locally on devices or just have them deleted once the player leaves.
I believe this idea will add greatly to cinematically interested people such as myself being able to make detailed short movies showing off the many variations of gameplay and unique graphics to communities, possibly making more people buy and continue to use Forza.
Another suggestion I have is to make visual PC-VR support, grab on your virtual reality headset, and then you can feel like you are right there in the car while driving it with a controller or a wheel setup, also I feel horizon-level adjustment and even seating adjustment for simulators would be great as well to not have a distorted looking view.
Also, I feel starting smaller as opposed to having a supercar instantly in newer Forza games would be awesome, like starting from the ground up, or even having greater penalties for crashing vehicles.
A mission idea could be something like a police chase as well.
