The Spanish Flu would still exist
My question is how much of the flu was brought to Europe from America, at least some speculation is that this flu ramped up in the American training camps and then crossed to Europe. Or was this a usual flu given power by starvation, troop concentrations and other factors in Europe itself? If the later we may see the flu still and we might see it cause a break in the war. But will hostilities truly resume after?
My arguments would be you need a war with a break in between, this requires more balanced sides and no easy victor, either early or late. Thus I suggest you first remove the USA, in fact make her less Entente leaning and trading with CPs to keep them in the game. Next I would take out Belgium and have the war open to the East, devolve to French offensives, having Britain hanging non-belligerent but pro-Entente. Better for the CP but still botched and fraught with imbalances so victory is not easy. I think you get a Russian loss by 1917 and exhaustion by 1916, especially economic, but not enough to end the war. Here Italy and OE have remained on the sidelines. War is a stalemate West and Germany is "winning" in the East. Armistice comes, something pushes everyone back to war, maybe an Italian attempt to get a cheap victory, or the outbreak of revolution in Russia, the war starts back up with the UK backing France, intervening in Russia, the OE going to war, A-H struggling against Italy and Germany trying to salvage its victory. Here you are trying to make the Great War work like the Balkan Wars, a series of wars with enough breather to let it fire up again. A war just as destructive but not as intense yet it sprawls over many years because the stakes are still high, defeat equals destruction, revolution or dismemberment.