It's almost guaranteed to be Gandalf. He was sent to aid Middle Earth against Sauron's reemerging power, which falls into what the show is portraying; he was reluctant to go, stating he was afraid of Sauron and weak; he took the form of an old man from the beginning to show humility; and he is strongly associated with fire (the "Secret Fire" of Eru Ilúvatar, and later the bearer of Narya, the Ring of Fire). He's even demonstrated his aptitude to talk to insects in the Jackson movies, in almost the EXACT SAME WAY as the Stranger talks to the fireflies.
Those are all perfect fits. Whereas for him to be either Sauron or Melkor, there'd have to be massive changes to the entire established lore.