The game engine is showing it's age and needs some fundamental improvements going forward. My suggestions, in order of priority:
Content (Aside from actual car models):
-Ability to upgrade/customize car interiors. I spend 99% of my gameplay in the dashboard view. Let me put some gauges in there or change the shift knob, or something. Feels weird to drive a fully weight-reduced car with all of the OEM interior still visible.
-Better wheel selection. The wheels in Horizon 4 are beginning to feel anachronistic....they feel very 2007 apart from the handful they've added in the last few years. The lack of more classic wheels is also a bit sad considering the growing roster of pre-70s cars. Not everything made before 1970 looks good on cragars...Also, the offsets are hilariously bad. How hard would it be to make it adjustable, or in the very least revert them to a "correct" offset for the car. It's a simple parametric model...I could do it!
-Ability to change exhaust models. I don't know why they haven't done this...it's a staple of car customization. Adding to this, the car shouldn't sound different when the exhaust is changed. They did this through FM2....it's kind of strange to do a full-tune on a car and the sound doesn't change.
-Physical garage - I like the implementation of the houses in FH4...somewhat. I think each house you purchase should have a set number of vehicles that it can store, and you should be able to walk around this storage area, akin to the PGR series...this could function as your 'favorites' in a way. There should be a generic warehouse that stores the rest of your vehicles by default...it could be an automated parking system where you can
cycle through cars on a conveyor to get around the limitations of displaying 400+ cars simultaneously...or it could be just a simple list. The houses added a *bit* of immersion, I'd like to see them go a little further.
-Better wings. All of the Forza body pieces are ugly as sin.
Tweaks
-Overhauled replay system. It's pathetic now. Barely suitable for the original Xbox, but not even good enough for 360 IMO.
-License plates should be car-specific, instead of globally applied to all your cars. It doesn't even make sense the way it is now.
-Improved menus. The interface is so damn clunky right now. Why does it take so long, take so many button presses, and feel so sluggish to auction cars? Why does everything take so long to save?
Gameplay
-Ability to filter drivetrain swaps out. I'm sick of every drivatar vehicle having AWD. Really takes the jam out of my donut to create a presumably all-fwd race and then have half of the drivetars bolting off the line with AWD.
-Ability to "project" a HUD onto the windshield of vehicles. I like going HUDless, but it isn't practical in-race. If I could have a mini-map + position/time projected onto the windshield, it would be more immersive while still providing me with useful info. I guess this is a stretch, and the implementation could be time consuming, but it would be a dope feature!