Think. The input frequency is a train of voltage pulses. A voltage, by it's very nature, is the potential difference between two points. The input to the hypothetical flip-flop has to have two points to enable it to recognise the input as a voltage. Same thing for the output.
That said, let's let our imagination run wild. If the incoming pulse train is in another form of energy, like light, and the input of the FF is a photosensitive device, and the output is another light emitter which is pulsed at half the frequency of the incoming signal after being divided by the FF. If the input to the subsequent stage is another photosensitive device, then we have a frequency divider without any electrical contact between the signal source and the following stage. Let's go even wilder and mentally devise an interface using ultrasound, RF, magnetic field, etc. and we have the same function as with light.
But I don't think this is the kind of thing you had in mind. With such a scheme, the FF unit will have to have its own power supply and a reference point anyway.