[JonesF1]

1. Restrict the power to say 500bhp.
No way. The more bhp the better.

2. Reduce the downforce by at least 75%.
The rear wings serve as billboards so thats not gonna happen any time soon. Im not sure it would reduce costs, if anything it would help prevent another R3. But a reduction in aerodynamic grip should happen.

3. Eliminate all electronic communication (including voice) from pits to car.
Voice, No. Data, Yes.

4. Allow any power plant, providing it used the specified fuel.
Combined with your first suggestion this would be interesting. Otherwise, no.

5. Allow energy recovery systems, such as regenerative braking: again, relevant to road cars.
Not really effective at this point. Maybe in the future?

6. Allow control of car functions by on-board computers.
F1 already has ECUs (black boxes).

7. Allow any transmission. Efficiency would again be key.
Basically everyone would be using CVTs. Why not? All F1 transmissions are full-automatic these days anyway. But manuals would be better for the sport IMO.