The other possibility was that the Stranger himself might be Sauron, but we are pretty convinced at this point that he is either Gandalf or Saruman. Nori insists that he is good, and he has certainly shown goodness and kindness. He could be Saruman, who was corrupted in later years but like Gandalf was originally one of the Istari, Maiar who were sent to Middle-earth as wizards specifically to help the peoples of Middle-earth to fight against Sauron.
But the Stranger’s unruly beard, rough grey woolen cloak, and his close relationship with the Harfoots are all pretty strongly suggestive of Gandalf. We also saw him contemplating the night sky while the line “not all those who [wonder or] wander are lost” was sung in episode 5, which was a line originally written by Bilbo for Aragorn in the books but which could equally apply to their friend Gandalf. In fact, Bilbo described Gandalf as a “wandering wizard” when he first met him in the first few pages of The Hobbit, so considering this episode opened with Nori explaining the concept of migration to the Stranger, we’re really almost completely certain that he is Gandalf at this point. Just watch them reveal that he’s Radagast the Brown after all, just to confuse us.
None of these characters match the disguise we’re expecting to see Sauron adopt, either – but this new figure, staring down ominously at the crater where Probably-Gandalf fell to earth, definitely does. In Tolkien’s legendarium, Annatar appears as a beautiful Elf, and while this character and their two companions are not dressed quite the same as the Elves we’ve seen, their look is very similar, especially the character standing to their right (and they are very beautiful, in an evil sort of way – in fact the leader looks a tiny bit like the Devil from Mel Gibson’s 2004 film The Passion of the Christ).
Another reason we don’t think Halbrand, Adar, or the Stranger are Sauron is that they are all embroiled in the wrong storylines. Halbrand has spent most of the series kicking about in Númenor, Adar is looking for something in the Southlands, and the Stranger is learning to speak and trying to avoid killing fireflies with the Harfoots, whereas Annatar needs to be heading to Lindon or to Khazad-dûm to try to corrupt Elrond, Gil-galad, Celebrimbor, and Durin.
This new character is a long way from Lindon or Khazad-dûm, but they are in a place it is logical for Sauron to be – investigating the Stranger’s arrival. The Wizards were sent to Middle-earth to combat Sauron, so it makes perfect sense that Sauron would follow the path of the comet seen in the first episode if he suspected one of them had arrived, to find out what was going on, who had been sent, and whether they were likely to pose a threat to him. Perhaps in the TV version of the story, it will be Probably-Gandalf’s arrival that prompts Sauron to work on the Rings of Power with the Elves and Dwarves.
On top of that, all the sound, imagery, and design of the scene in general are just screaming “This Is Sauron!” The crater where Probably-Gandalf fell almost looked like a giant eye for a moment, as the image of it in flames faded away. The music sounds like the Black Speech of Mordor. The new character has blackened fingers and carries a staff that looks like a prettier version of some of Sauron’s weapons, and they are obviously very powerful.