Sauron is a highly significant character whose later existence plays a substantial role in The Lord of the Rings mythos.
For context, Sauron is the Dark Lord that secretly forged the “master ring that controls all others,” as noted in The Fellowship of the Ring’s prologue.
Fans would love to see the character in another form beyond the Lidless Eye appearance, but it’s hard to say if that’ll happen.
That is the same ring that Frodo tries to destroy, of course.