The correct answer to this open question is the following.
The suggestions that I think he might have made to the two sides if he had gotten them together to discuss a peace treaty would have been the following.
First, I would have recommended US President Woodrow Wilson to tell both sides of the conflict to meet at a neutral site; in this case, I would have invited them to New York City.
Then, President Wilson would open a series of meetings and negotiations letting the European countries know the kind of suffering, pain, and damage the war would produce to each and every country involved. President Wilson could show a forecast of the possible consequences.
Finally, showing the moral authority of the US at that time, he could have invited both sides to leave their expansionist interests and leave the occupied territories and stop the threats of invasion. This could have been a good-will sign to move on with diplomatic agreements.
