If the aim is to have relevance to today's (and tomorrow's) road cars, I would say that only internal combustion engines should be allowed.
The primary goal today, at least via lip service, is to becoming more "green". There are myriad problems with alternative motive power that have yet to be resolved: how do you safely dispose of batteries (which is true of both electric and hybrids) being a HUGE one. Hydrogen power is laughable as far as energy efficiency is concerned: the main way to produce hydrogen is thru electrolysis of water which requires large amounts of energy. Then you have to separate the hydrogen from the oxygen, and THEN supercool, to cryogenic temperatures, the hydrogen to allow it to be mobile enough to use as a motor fuel (and the cryogenic tanks weigh a TONNE!).
With electrics, you not only have the problem of disposal of the batteries, but you are only moving the source of the pollution from the car to the power plant, and, since no energy generation process is 100% efficient, you lose out that way as well.
I would say that the fuel limit per race should either be in kilocalories to equalize gasoline with diesel, etc.
As far as aero goes, what I would propose is that front wings cannot extend beyond the inside wall of the front tyre and are limited to two elements. I would also ban raised noses. The anteater look isn't even appealing on anteaters--it is far worse on race cars.