If I were the rulemakers, I would simply wind down the downforce to about half its current level BUT free up other technologies such as drivetrain, engine, suspension and tire technlogy a bit more.
Basically, my intention is to force aero development towards efficiency (L/D) instead of just more and more downforce. This is both more enegry efficient and mouch more relevant to road cars.
As for allowing lower profile and stickier tires, this is to relieve overdependance on aero generatwed grip and more towards mechanical grip. This reduces overtaking issues by allowing great speeds without causing massive turbelant wakes that make a joke of overtaking attempts. The stiffer tyre sidewalls would also put emphasis back into suspension design and bring F-1 away form current no travel hacks that rely on sidewall deflection for as much as 40% of its travel. As I said before, what would you prefer, a consistant srping/damper suspesion or a sidewall that changes all the time?
And engines would have only 1 rule: A power limit. All engines shall not make more than a specified amount of power (say 750hp). This again puts emphasis back on increasing L/D, since the power limit prevents tthe use of more power to cope with more drag. This is all relevant to road cars too. How you achieve the power is essentailly free, so many types of powerplants and fuels would be allowed, again of relevance to road cars as it allows the development of next generation powerplants. The peak power limit also forces more energy efficiency as this would reduce the need for pitstops. The intentionis to force the development of superior drivetrains to extract as engine power and as efficiently as possible and put emphasis on the generation of broad torquebands. Let's make F-1 cars throttle steerable again! With the current peaky powerbands, throttle steering isn't viable.