Angry hill troll; the idea of Eru Illuvatar has neccessarily been in the conception since just about its beginning. This itself is insightful into God, something that is augmented by the end-incarnation idea.
When this came into Tolkien's writings, I don't know. Someone else had probably better answer this. Notwithstanding that it may have come in later, although I seem to recall it in the Lost Tales in some form, the idea of the God Illuvatar is strong (not always obviously present, but that's the beauty of it. Sorry to sound like a television literary critic there), powerful and insightful in its entirety.
Finwe - I can ascertain one thing, and that is that the idea, that of destruction of the world in order to destroy Melkor, comes from the beginning.
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And it is said that he shall destroy the world to destroy his foe (Morgoth)...
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That is from the Lost Tales. Since Melkor has in fact managed to get into the very fibres of the world, he cannot be vanquished until it is as well; or at least unMarred, but that is an act that only Illuvatar can, perhaps, perform. But the allusion seems to be that he will break it. We can't determine the specifications of the Last Battle beyond the allusions and premonitions-written that we have - I tend to say one thing 'wait and see', to that.