To be just a very little realistic, my proposal is gonna consider some elements of 2021 rules but I am gonna be very restrictive:
Overall Spec:
1. Floor (Very important area because of Ground effect and diffuser)
2. Bargeboard area
3. Crash structure (Powerunit/Gearbox manufacturers need to figure out how to accommodate with the spec crash structure)
4. Except Powerunit and Gearbox, the rest of the mechanical/non mechanical components (Like suspension, steering, dampers, wheels, brake ducts, cameras, mirrors and ...)
Manufacturer based Spec:
1. Manufacturers should supply their customers with the exact Powerunit and Gearbox (If a team wanted to produce their own Gearbox, no problem!)
2. Manufacturers that supply PU, also should supply the exact engine cover and sidepod to their customers.
Areas that teams can develop throughout the season:
1. Front wing
2. Rear wing
Limitations:
1. The surface of the car should be slick with no element coming out of the surface.
2. There should be performance and weight cap for all the Powerunit/Gearbox manufacturers (Fuel and lubricant included)
3. Teams should use CFD instead of Wind Tunnel.
There are lots of areas that I haven't covered because I am not a rule maker, anyway!
Good Luck! 