[octothorp]

So here's my (likely wrong, but hopefully plausible) Halbrand theory:

Spoiler!

There are strong Sauron-adjacent hints to Halbrand, but it's hard to reconcile his actions (saving Galadriel, trying to get a job as a lowly blacksmith, advising the Numenoreans on going to Middle Earth to fight the orcs, encouraging Galadriel in her quest to destroy Sauron but also pulling her back from darkness), with the actions of Sauron.

I think he is set up to become a Nazgul, but more than that I think he's already had exposure to Sauron whether he knows it or not. If I was to try and make a plausible backstory for Sauron immediately prior to the known history of becoming Annatar, he would likely be testing the craft and magic necessary to create the rings of power, as well as their effect. Let's say he did so in a backwater kingdom in the Southlands, possibly forging another ring or other embued object, some sort of proto-ring. And suppose he took a talented apprentice while doing so, whether that apprentice was a commoner or a member of the royal house. The effects of the proto-ring were as Sauron intended, leading to betrayal. Somehow, the proto-ring ended up in the hands of the young apprentice, Halbrand, who still carries it in a pouch). All of this likely happened prior to his encounter with Adar. It's even possible that there could be reasons why he blames Adar for the loss that Sauron inflicted. He shares a commonality with Galadriel because they both have a loss at the hands of Sauron, so their vengeance is more alike than they know.

Like I say, this is probably wrong, but I think there's are much more creative and interesting ways to reconcile Halbrand's actions and the symbolism around him than him simply being Sauron.