I disagree that Sauron was greater than Morgoth, for it twas Morgoth that defied Eru, that sought worship and that created the abominations: orcs, belrogs, fire drakes, and etc. Sauron did indeed do great evil, especially in Numenor. However, Morgoth in my opinion is the greater dark lord, for he sought to replace the Creator (Sauron attempted, but lacked the powers.)
I think Morgoth's return to Arda paralles Revelation in the Bible, when Satan is to be released upon the world after a millenium. Morgoth likely is to ravage and attempt to lead astray/tempt the Children of Eru before the final judgement.