200 litres per race, no refueling, any engine you want!
I don´t know if that is 1 rule or 3, but they are linked.
That would allow the following:
1- A lot of uncertainity till the end of the race, you´ll never know who is gonna stop 1 lap before out of fuel!!!
2- Creativity in engine design
3- It is a "green" rule that will enable efficient engines and can be linked with automotive industry, wich is one of the things it needs (efficiency)
4- Different pace from the begining to the end of the race, with fast lap times at the end
5- Balance of the car could change from full to empty, making races more unpredictables
6- Drivers will need to administrate fuel consumption, so the winner will be the faster and more intelligent driver
7- Manufacturers have enought money to pay for development, if they don´t like it, screw them, if they all go away and just one team remains spending what others can´t, redo rules again!!!
Now you can post other benefits from this spectacular rule!

"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio
"I have no idols. I admire work, dedication and competence." -Ayrton Senna