Alright, calling it now - if I'm right I'm gonna do something CRAZY, I don't know what yet, but I think this is how it's going to go for some plotlines of s3:
I think the Dark Wizard is the one who kept Celeborn prisoner during all this time - namely because the list is getting pretty thin at this point. Morgoth? Maybe at some point, but he's gone. Sauron? Busy being a pile of forbidden ramen noodles. Adar? You'd think he would have told Galadriel about it, or that there would be a hint before he died. The Silvan Elves? Celeborn is Sinda, not Noldo - they wouldn't imprison one of their own. The Dwarves? I mean, sure, the Sindar and the Dwarves don't really like each other, but why keep a single Elf prisoner for a millennium? I also don't see him just... chilling somewhere - dude is MIA and presumed dead. You'd think he'd give a sign of life in the few 1,000 years he was gone, and deadbeat husband, he was not (I was also correct to assume at the beginning of s1 that Celeborn was presumed dead before the reveal in ep7, because no way Gil-galad would have just sent Galadriel to Valinor willy-nilly if she had a husband and daughter in the neighborhood).
So honestly? The Dark Wizard is pretty much the only one left standing. Where is he? Rhûn. Far enough that no one from Lindon or Eregion would take notice. Who's heading to Rhûn in s3? Sauron. Who's also probably hanging around Rhûn? Gandalf.
You kind of need to establish that Celeborn and Gandalf are friends, too. And if you want the reunion between Celeborn and Galadriel to be even more meaningful, it would make sense for Celeborn to also have a temptation of his own. Of course, Sauron would go for a very angry and bitter Elf who feels trapped - that's his toxic dating trait.
I'm also suspecting the Dark Wizard is the future Witch-King, because a) if you want to get very literal, he *is* a king of witches; b) there is a precedent for one of the Istari to be a Ring-bearer (Gandalf in the 3rd Age); c) it would make a lot of sense for Sauron to choose someone of stature to lead the eight others.
Also, it would be PEAK COMEDY for Sauron to have a thing for this very angry grubby Elf, while not knowing he's his ex-situationship's long lost husband, basically being the devil on Celeborn's shoulder, while Gandalf is pretty much the angel on the other side. And that would be exactly the kind of setting that would make the general audience: "Oh, Galadriel married THIS GUY? Yeah. Totally makes sense."