I would like to see Formula 1 just set a limit on fuel used in a race (but not flow) and let the teams sort out how to get to the end on that amount of fuel....
I think this would be very bad for racing. I'll try to explain why. Given the huge advantage to be gained by having large peak power (qualifying, overtaking, start,etc.), constraining only the total fuel (or energy) used during a race, will lead to the teams designing engines that at peak operation will consume the total fuel allotment in a (small) fraction of the race. Consequently, the actual racing will be dominated by the drivers having to manage their fuel allotment. This means lots of lift and coasting, drivers losing the race because they ran out of fuel, none of which sounds particularly interesting to watch.
A secondary concern would is safety related. Regulations in the last decades have had to limit the engine power available on safety grounds. A total fuel used limit, does not put an effective limit on the instantaneous engine power. This could lead to unsafe situations.
My vote would be to limit fuel flow (or energy consumption), and nothing else. This will allow the engine to be driven at maximal power, for as long as they stay in one piece, while effectively limiting the maximal power.