A capacitor, by definition, has two conductors mutually isolated.
Having a single wire, totally isolated from everything else, including ground, is pretty much just as good as having no wire at all. You can send a signal using electromagnetic radiation but you don’t need a wire for that. If you want to have an actual flow of current you need to have two conductors of which one can be ground and the other can be any conductor isolated from ground. That does not detract from the concept that you need two conductors.
Coax cable has two conductors of which the shield is “at rest” and the central conductor “does all the work”. In other words, you do not have currents going in opposite directions but only the central conductor carries AC. This is called an “unbalanced” line.
With parallel conductors both conductors conduct in opposite directions and this is called a balanced line. To adapt a balanced line to an unbalanced line you use a balun.