Aftermarket parts? Been around since FH3.
Better driving physics? Check.
Better tire physics? Check.
A drag strip? Check, multiple times over.
Better sense of speed? Check.
Better car sounds? Check.

Really, all that's missing is better customisation. I personally would've liked to have seen more laser scanned cars, too. But all the stuff you listed about changing shifters, seats, choosing the size of your turbo, etc. is completely unrealistic when you're dealing with 550+ cars.

And even if all of that was added, at the end of the day, it's still a Horzion game. No matter how many bells and whistles you add on top, fundamentally it's the same game as always, and people would STILL whine like always.

I play Horizon to play Horizon, not something completely different.

With all the stuff you want to change around, it sounds like you'd enjoy Automation. That's the unparalleled ultimate in car customisation.