What does Caliban hope to accomplish by his plot againest Prospero? Why does Shakespeare include his subplot mirroring the conspiracy of the nobles?

Caliban wants to rid himself of Prospero. He essentially wants to trade one master for another he does not yet know well . Prospero was usurped by these same people on the mainland and we see the same forces at work again. This, however, is Prospero's island, and the magic about it (Ariel in this case) prohibits what happens in the temporal world to happen here. This sub plot shows that the nobles have still not learned anything despite their near death on the seas and their time on the island.