Okay, it has been established on this board that if the US of A did not join either side in WW1, then no-one would win. Firstly, let's have a look at how long it would last.
Germany collapsed in early November 1918 with America, ittl they might make it to Easter 1919 but definitely not to June 28.
France almost collapsed in 1917 OTL, and with no America they could collapse pretty much anytime in 1918 or 1919.
Italy was in no danger of collapsing whatsoever.
Austria-Hungary was fine until Piave River and Vittorio Veneto when its collapse became inevitable.
Russia collapsed in 1917.
Not much would change with the Ottomans as their Empire rapidly disintegrates over the course of 1918 and 1919.
Britain were one of the more stable countries and could probably last until 1920 but not until Easter 1920.
Japan and the various Balkan countries would not change.
This means that a negotiated peace is likely to occur in March-May 1919. France, Britain and Italy would be in no position to do a Treaty of Versailles. Alsace-Lorraine likely ends up with whoever currently holds it or is less collapsed. Italy comes out pretty well and the UK take some colonies.
Now, what happens next?
France will probably be even more revanchist than they were from 1870-1914 and I could even see not!Fascists taking control.
The pro-war Liberals and Conservatives get discredited and I can see MacDonald having a free hand for most of the 20s. I'm not sure whether he will be an early Attlee or not. The Tories will recover eventually but I'm not too sure about the Liberals.
The Kaiser still falls and the chaos of Weimar still happens. With no Versailles, lot's of the angry young men who became Nazis IOTL like Goebbels, Hitler and Himmler may become Commies. Alternatively, Hindenburg or Ludendorff could put an end to the chaos.
Italy is in a stronger position than they were otl and do quite well in the 1920s. Benito Mussolini is an obscure Socialist politician.
Japan are still rising, China is still chaotic and the USA is now the most powerful country in the world.
North-Eastern Europe is a complete mess. The Poles revolt against Germany and perhaps the Soviets join in the fun. This will lead to a horribly violent three-way war in the early 20s between the Germans, Polish Rebels and the Soviets. No idea what happens there.
World War II is practically inevitable now.