The Dark Lord does like to be dramatic (see also: the giant flaming eye that will eventually sit atop Barad-dur) and this is certainly the most dramatic entrance any character has had in the series thus far. I mean, none of us would expect Sauron to do something as common as take a boat back across the Sundering Seas to Middle-earth like some elf, would we?
It’s already very clear that the driving emotional engine behind much of The Rings of Power is Sauron’s inevitable return. Yes, he was defeated when Morgoth fell at the end of the First Age, but as a Sith apprentice must always rise when their master is defeated, we all know he’s coming back. This would be a dramatic way to reintroduce the series’ greatest threat right under viewers’ noses and establish a connection between Sauron and the ancestors of the very beings (i.e. hobbits) that will one day mean his doom. Plus, it can’t be an accident that the fire crater his crash landing creates looks an awful lot like the Eye of Sauron, can it?
Gandalf the Grey
Despite the fact that The Rings of Power is, for the most part, boldly charting its own path through J.R.R. Tolkien’s lore, no one can tell me that if Amazon saw a chance to somehow incorporate one of the franchise’s most popular characters into this series that they wouldn’t take it. And I can’t say I blame them, even if the Stranger’s presence doesn’t entirely mesh with the existing lore surrounding where Gandalf came from or when he arrived in Middle-earth. (Details, right?) But since Gandalf is one of the Maiar (angelic, primordial spirits), who were known to occasionally visit Middle-earth to check up on things, it’s not impossible.
For starters, the Stranger basically looks like a crazed wizard and his strange abilities (that booming scream, his affinity for talking to bugs, writing in symbols) all seem to suggest he’s a being with some kind of magic. His initial connection with the harfoot Nori could foreshadow Gandalf’s own longtime interest in and connection to the hobbits that will one day be their descendants, and his fall to earth encased in fire is eerily similar to Gandalf’s fall through Moria with the Balrog and his subsequent resurrection afterward.
Bonus points for the fact that the Stranger is impervious to fire, which happens to also be Gandalf’s Maiar alignment. (He will eventually wield the elvish ring Narya, the Ring of Fire, in the future.)
Though viewers of The Rings of Power will be familiar with several other members of Gandalf’s Istari order—fellow wizards Radagast the Brown and Saruman the White—it’s unlikely that either of them is the Stranger, simply given what we know about their respective stories and histories. Saruman’s web of deceit and betrayal is complicated enough without thrusting him into this series, and the Stranger doesn’t seem in tune enough with nature to be Radagast, though he’s got the borderline madness thing down.