What you are really asking is the old theological question: "What if the Fall never happened?" To which there is no reasonable answer, because it did.

Please note that Melkor (like Lucifer in the Bible) had fallen long before the creation of the World. He disrupted the Music of the Ainur, and the Arda that was subsequently created included the effects of his disruptions.

If you wish to explore the theological aspects of an averted Fall, I would recommend C.S. Lewis's novel Perelandra. This is the second in his Space Trilogy. It explores what happens when the Adam and Eve characters on another world are tempted by the Devil but reject the temptation. (Also note that the Adam and Eve characters are named Tor and Tinidril - Tolkien was convinced that Lewis stole the names from his Silmarillion stories: Tuor and Idril).

Also, on a more academic note there is an interesting theological essay on the possible role of Christ in a non-Fallen world written by Prof. Loren Wilkinson of Regent College, Vancouver (an Anglican or Episcopalian seminary) in Doing Theology for the People of God: Studies in Honour of J.I. Packer, eds. D. Lewis and A. McGrath, Inter-Varsity Press, 1996. The essay is entitled: "Immanuel and the Purpose of Creation."