Cable based data transmission without common ground
The real one wire transmission line is Goubau Line. See the patent:

https://patents.google.com/patent/US2685068

In the patent it's applied as low loss transmission way between microwave radio transmitter and its antenna:



Essentially the field between the mid wire and the shield of the coaxial cable is let to spread to fill the space. Only the mid wire is needed to guide the wave. At both ends there's a matching horn.

NOTE: there's no hidden second line such as the ground. The antenna tower can be any material.