They're pushing hard to make you think the Stranger is either Sauron or Gandalf (or even Saruman or Radagast), but those seem to be a bit on-the-nose, so I'm gonna swing for the fences and say it's gonna be one of the previously unseen Blue Wizards, Alatar or Pallando.
The time that the Blue Wizards arrived in Middle-earth is uncertain. In Unfinished Tales, Tolkien wrote that the five Istari came to Middle-earth together in TA 1000. However, in The Peoples of Middle-earth, they are said to have arrived in the Second Age, around the year SA 1600, the time of the forging of the One Ring. Their mission was directed at weakening Sauron's forces in the eastern and southern parts of Middle-earth, whereas the other Istari were focused on the west.
"I think that they went as emissaries to distant regions, east and south... Missionaries to enemy occupied lands as it were. What success they had I do not know; but I fear that they failed, as Saruman did, though doubtless in different ways; and I suspect they were founders or beginners of secret cults and "magic" traditions that outlasted the fall of Sauron."
—J.R.R. Tolkien
https://lotr.fandom.com/wiki/Blue_Wizards
The bolded parts all fit with the general (compressed) timeline and regional setting of the series and where Sauron's forces are gathering.
When Gandalf returned from death in The Two Towers, he experienced a less severe but similar disorientation and memory loss, so that might be the way Istari arrive in Middle-Earth.
He can communicate with and control insects, like Gandalf did atop Orthanc and in the Battle of the Black Gate, and presumably all wizards can do. Although the fact that the fireflies all died immediately after he used them is very Sauron-ish.
When he picked up the stick to draw symbols in the ground he was unintentionally using it like a wizard's staff and hurt Nori's father some distance away.
He did the raised "drown out all other sounds but my own voice" thing wizards can do.
Points in comparison to Sauron is that he arrived in flames and the flames burned cold (not hurting Nori), just like how the torches in the ancient ice fortress in Forodwaith burned cold because Sauron had been present there. But maybe that's just a general Maiar trait shared by Wizards and Sauron.