It's either Gandalf or Sauron. And if it's Sauron, he's appearing in the form of Annatar, the kindly "Lord of Gifts" who deceives Celebrimbor into forging the rings of power.
Both of them are associated very strongly with the element of fire. Gandalf's element is fire, and Sauron seeks the fire of the forge. The very first thing the Stanger asks, upon waking up from his impact, is "Where is my fire?"

In all likelihood, the writers will tease out his identity for the remainder of the season, and possibly even longer. But a lot will be revealed by where, precisely, the Stranger wants to go. If he's itching to get to Khazad-dûm and/or Eregion, that's Sauron's agenda. If he wants to go to Lindon, where Sauron is not permitted to enter, then it's Gandalf.