In Douglas Adams's Dirk Gently's Detective Agency, all of Bach was written by aliens. Despite there being lots of evidence of Bach's life (and actual descendents!) I find this emotionally an entirely convincing theory, because Bach just is so much better than all the music composed by humans before or since.

I can therefore sort of see where the Shakespeare conspiracy theorists are coming from, Though I think "an alien" or "second coming of Christ" would serve them better than Bacon or Oxford.