Something's up with Halbrand, but I don't buy that he's Sauron. Most of the supporting evidence for it is cute rather than actually well-founded, like the fact that he knows how to smith. There's a good number of stories he'd be a much better fit for; I for one am rooting for him to be the Dead King, which would be a much better gut punch after sufficient buildup.
That said I don't have a *better* candidate. Meteor Man is almost certainly a wizard (hopefully Blue rather than Grey), none of the Numenoreans really have an in. My best guess is something to do with an offscreen Elf around Lindon (remember that one top-secret meeting that Elrond wasn't allowed into? Smells like
Plotting, and not just Gil-galad's). Adar also isn't eliminated as an option, since, y'know, the dude could just be lying and/or assuming Adar's place.
Or maybe it's Poppy. Did you know she put itching powder in that one lady's cream? Diabolical.