Sure! One thing first though; I can't see your post while typing, so I might bot remember all of the characters you mentioned. Also, just somethi to note; the upcoming series is set during the second age, which lasts around 6000-3000 years before LOTR.
PS: I'm typing this from a phone, lest I'd just take a screenshot and save in another tab...
Sauron: Very likely. He is actually the main known antagonist of the Second age, and the Elves fight him for quite a long time, and at a certain point, he meets the men of Númenor (if you don't knoe of Númenor, you might want to look it up, or you'll learn of it from the show.
Gandalf: If I remember correctly, there is debate concerning when both Gandalf and Saruman along with the other wizards show up, but hopefully, yes, Gandalf (and Saruman) might show up.
The tree people, or the "Ents", did absolutely exist during the set time of the show, so hopefully they'll show up as well.
When it comes to the orc you mentioned, I suppose you mean Lurtz, the orc who shot Boromir. If I'm correct guessing so, no, he won't appear. During the FOTR (fellowship of the ring), you watch as Lurtz is "created" by Saruman, though if I'm not wrong, Sauron might have created orc similar to Lurtz (Uruk-hai) that might appear.
Nazgûl: Probably, as they were actually made into wraiths at some point during the 2nd age; corrupted by the Nine Rings of Power.