Detail 181: An Adposition

A simple, subtle, yet slightly unusual adposition ­­­­– whose existence in some actual natural language you can take for granted – that struck me as an idea today, and which never has occurred to me previously, is this:

An adposition that marks subjects, but only if they're in unexpected positions with regards to word order.

Where this might get interesting is if the language has transformations that change word order, i.e. fronting the verb for polar questions. Should the new position be considered the expected location of the subject in questions? Both ways seem reasonable: verb fronting dislocated the subject, but with regard to the clause type, the subject is very much where it should be (it's rather the verb that has been dislocated).

What further could get really interesting about this is adposition stranding, as well as intransitive use of the adposition (i.e. no noun in the entire clause which could be its 'subject' so to speak). It could of course also have some kind of restriction like never appearing with intransitive verbs, making the language slightly more ergative – or never appearing with existential verbs? Or some set of verbs with which it appears even when the subject is where it should be.

Further, one could imagine it being used with nouns uttered in isolation, e.g. when used as answers to questions and the like. Maybe some pronouns have incorporated it into their forms, or maybe pronouns are immune to its effects – i.e. never are marked by it.