I think it would butterfly a lot of the character of the interior of Mordor and its approaches, the descriptions of which were probably mostly based on his experience of the Western Front. The same applies to the areas dominated by Morgoth/Melkor in the First Age. Possibly the huge armies of Orcs and the human allies of Sauron or Morgoth would also not feature as much.
OTOH, I believe he had already begun the development of the Elvish languages before the war, which might have resulted in a Sillmarillion analogue to provide those languages with a history. If he had created such an Elvish history, he would probably have also created a story similar to The Hobbit for his children at some point, loosely set in his Elvish world.