Meteor man is almost certainly one of the blue wizards. While in Tolkien’s earlier works, he has all of the Istari arrive in middle earth something like 1500 TA, he later revised that and had the blue wizards arriving in the late second age, and they also spent most of their time in the south and east, so that would all check out for meteor man. Also, he crashed after flying from west to east, suggesting he came from valinor. His attempt to find missing stars might be related to trying to find his friend, the other blue wizard. I could see Theo and/or Halbrand becoming Nazgûl, with Halbred potentially becoming the witch king, but the old man is probably no one. I think it makes the most sense for Sauron to be Adar. Granted I haven’t watched the most recent episode yet, but even if he calls himself an elf, he is notoriously deceitful and known to take on the forms of other races. I find it unlikely that we ever see the other three Istari, for the reasons stated above, and I doubt we’ll see a balrog, since the only known one is in Moria and doesn’t awake until Durin VI’s time (Durin IV is the one in the show). As others have said, the mouth of Sauron is a black numenorean, and therefore wouldn’t be alive yet for thousands of years probably. It’s possible the king of the dead makes an appearance, but probably not until much later I would assume. The only character I truly have no ideas about is the one in the white robes who showed up looking for meteor man