. . . If an outbreak started in 1916 . . .
Baseline answer is that it changes everything. The war ends in more of a stalemate, we don't have a highly punitive Treaty of Versailles, and there's
no rise of a Nazi Party in Germany.
Now, utopia, high trajectory, freedom, democracy, prosperity, growing and vibrant middle class — often very boring to read about!!! In fact there's a quote from the philosopher Jeremy Bentham to the effect, Wars and storms are best to read of, but peace and calms are better to endure. And then there's the urban legend of the Chinese curse, 'Interesting times' [May you live in interesting times]. Yeah, probably not a real Chinese curse, more something just continually repeated on the business lecture circuit. But the point is very taken.
Okay, so, (1) find a way to make a utopia interesting, and I mean, a real utopia, not just a control society called a utopia. This might make for an interesting writing exercise, but I wouldn't recommend more than a couple of weeks, because it's just too likely to be stilted, formulaic, cardboard character, and just not the best freely written art [reviewed days or weeks later with fresh eye!] of any of us, or
(2) mixed bag, just like real life. This is often the best fiction. The challenge of course are the characters who act across the terrain, or
(3) a dystopia (and I think it's fine for a writer to do his or her own take on something already done, just in that case, go faster on the set-up, it's all about the characters anyway)
