The rules in F1 are actually quite constrained. The key differences between F1 and Indycar or NASCAR is the materials and level of electronic control that is allowed. For example, an F1 ECU now controls the engine/clutch/gearbox/differential/fuel system/KERS/DRS.

This is hardly a min/max formula (like an open GT racing class where anything is allowed and the only criteria is horsepower to weight ratio), instead it is a formula that promotes the development of certain technologies (primarily electronic) to gain every 0.001 of a second per lap. Otherwise, the spec's are just as tight as Indy and NASCAR.

marginally. They had a LMP with Penske. That was interesting.
But some people really are die-hard Porsche fan boys the way the tifosi are.
I suppose having Merc, Ferrari and Porsche in it would rile up the fans.
But would that make for more interesting races? Highly doubtful. What's another billion spent atop the already spent billion going to add? These costs are insane.

What F1 shoud do, and people have been saying this for ages, is put those billions into a formula that brings back the emphasis on mechanical grip instead of aero. Get rid of the ez shift and bring back engines and transmissions that highlight driver skill.
Watching Nelson Piquet or Senna drive one-handed through Monaco with 1000 HP at their finger tips is why qualifying was standing room only. Those onboards make the hair on the back of your neck stand up.
Now its like watching an Xbox game.

But having an aero grip formula makes for predictable races where 1 or 2 teams wins most weekends and that makes it easier to price sponsorship up and down the price chart. The last thing F1 wants is for the human element to be deciding race outcomes, that migh actually produce races where you're not falling asleep by lap 30.

Porsche stated had it not been allowed to develop an LMP1 car to compete against Audi, they would have considered F1 as an alternative; but their first priority was going back to LeMans.

Sadly, there's no place in the states for P1 cars to race and we get Daytona Prototypes (really wanted EMS to win Sebring, but the Nascrap fix was on in the end).

6 Hours of circuit of the Americas
WEC is scheduled to be in Austin on Sept 20.

TUDOR Championship is scheduled to be there the same weekend.
IMSA | TUDOR United SportsCar Championship | Circuit of The Americas

As soon as event schedules are finalized I'll be going. I think this is the only chance we get to see all three Factory teams from Porsche in the same weekend.
LMP1
Team Mathey
Porsche North America

Porsche races to win and the experience of new (or returning) marques in F1 highlights how difficult, if not impossible, that is out of the box. While Mercedes eventually came to have considerable success as an engine manufacturer, it is has taken years for them to become successful as the builder of an entire car. BMW also found that their efforts at producing both chassis and engine were unsuccessful and bowed out, albeit after expending countless millions, if not billions of dollars.

If Porsche had been interested in a return to F1, the time would have been 3 years ago when the new formula was announced for this year. Their experience in developing high-performance hybrid systems with engine mapping and energy regenerative braking for the new 918 would likely have helped to defray at least some of the development costs of a new power plant for a Formua 1 car. In addition, they would have been starting with a fresh piece of paper at the same time as Mercedes, Ferrari and Renault and they would not have had the distraction of having to field competitive power plants running to a very different formula during the intervening three years. IMO, the time and place for a return to Formula 1 by Porsche has truly come and gone.

Make no mistake - Formula 1 is still relevant and I suspect that the worldwide audience will increase under this new formula. I also suspect that over time, the advantage now being shown by the Mercedes powerplants will be overcome by the other engine manufactuers (although I am not convinced that it will occur this year - especially if the disadvantage is in the gasoline engines, as I believe their design is effectively frozen for the year). Will Renault/Red Bull bounce back as quickly as some here are predicting? Although their chassis/aerodynamics seem first rate, that is something which can be changed/improved over the season by other builders.

The performance of the Renault at Melboune is still suspect, IMO, to the extent that they were not running in accordance with the specified fuel-flow limitations in the formula. We must remember that overall fuel consumption was not a problem for any of the engines (and if that was ever in doubt, it was ensured by the number of laps under full-course yellows). By exceeding the maximum fuel flow there can be no doubt that Red Bull were able to improve the output of the engine. How competitive the powerplant will be when running to specs remains to be seem (although their may be some evidence of that in the car driven by Vettel, which apparently ran with the approved metering device). If so, they are not out of the woods yet.

Brad