Theres many good ideas in this thread. But may repeting themselvs.
The one idea that is that downforce needs to be futher limited, but more engineering solutions need to be freed up.
The idea of the LMP1 style short chord rear wing is the one that i personally like, as it reduces drag and wake alot, making cars follow closer, needs CFD and wind tunnel testing however. If this were set up, id like to see the rear wing go back to the width it was in 2008, with a set of slightly larger rear wing end plates as well, as the commercial departments wouldnt like that idea of such a small area for advertising.
Id like to see a pretty much standardised, and heavily regulated centre section, as long as there was no appendages on the sidepod rears, as this way i think there is a way of reducing downforce more. Standardised centre section for front wing, rear wing, diffuser, mirrors and air box cross section (reduced by 20% from current). Also make exausts come within this sentre section as well to reduce turbulence in the extremities. Im thinking 600 or 700mm for this centre section. In this area the floor must be completly flat to arround 750mm forward of the line from the front of the air box.
Engine cooling should also be more standardised, making the sidepods a regulated size making the cars able to run closer, for longer behind another car meaning that cars engines dont overheat.
Id make it that front wings have to have no more than 3 elements, with multi element shaping, meaning that you can have one element at 12 degreese and the higher one at 9 degreese for example, making it that the car can be faster in a straight line. Also allow unlimited FFA adjustments, and have them linked to the multi map as well.
The consensus is to also increase mechanichal grip, bigger tyres would help lots, and the re-introduction of wide track would also help, but personally, id leave the track alone, but make tyres bigger would be a given. A single tyre company would be appointed, where they would have 3 spec dry tyre compounds, of whitch there would be 3 sets of each given to each driver for a race weekend, the set used at the start of each quali session needs to be used througout that session and carried into the race. Also ban tyre warmers as well.
Power steering would also be limited, but car weights brought back down to 605KG, meaning that smaller guys would be great for weight purposes, but bigger guys may have the stamina to last the longer races better. Of whitch id have five 500Km races a year, but also have five 250Km races a year, the rest would be 350Km races.
One other thing id do is limit engines to 800bhp, open development on the current engines for a year, id also bring in turbo boosts as well, limit them to a useabe of one single 7 second burst per lap, but turbo boosts are limited to 10% of the race distance laps also allow the RPM limit to be raised to 19,000RPM for a maximum of 20% of the P1 lap time for the race. So if there is a 80 second lap, then 16 seconds of the lap can be used for the RPM boost, but if there is a 57 lap race, there can only be 6 turbo boosts. The laps boosts are used, RPM limits cannot be used and vice versa.
Id also limit fuel to a maximum of 130KG for the race with a single 20KG top up avalable in a fuel only pit stop from a NASCAR style fuel filler, if the driver needed it. Teams could use this in their strategy as well. Id also allow Diesel powered cars, but their fuel is limited to 95KG with a 12KG top up avalable. They would be limited to 720HP engines and 5 speed transmissions.
In thinking in my own little world here, but personally, i think that it could work. Meaning that cars have less wake, more mechanichal grip ideal for overtaking, but the green credencials. Creating performance differencials and more competition, not only between teams, engines, designers and drivers.