As far as I know, basically by definition, subordinate clauses must be embedded in a matrix clause. Therefore, if sentences of the type "¡Que le vaya bien!" are subordinate, where is the matrix clause? Some have said that such constructions result from the omission of a main verb such as espero (i.e., "¡Espero que le vaya bien!"). Would Spanish speakers interpret the two sentences as equivalent then?
On the other hand, if they are not subordinate, what is the function of the subordinator "que"? How should its presence be analyzed?
I've looked for similar examples of subordinate clauses being used as complete sentences. The only one that came to mind is "If only . . . !" in English, but the Cambridge Grammar of the English Language says that in this case, it's just that "the apodosis is omitted."
It also occurred to me that the "que + subjunctive verb" construction is similar to "would that . . ." in English, but I think it's clear with "would that . . ." that there is indeed a main verb with an omitted subject "I."