If you take away the great depression, the social fabric of especially Europe may not tear as badly; hence some of the reasons for WWII may go away?
If we look at WWII as "unfinished business" from WWI, then you need go back even further.
The interesting part is if you can avoid WWII with Hitler. That I am not sure about. With Hitler at the helm, it was unavoidable, at least the war with Russia.
The rise of fascism was closely linked to the social conditions and that is a bit difficuolt to do something about.
HOWEVER: If you really want to get into overdrive:
More wholesale slaughter and worse Spanish Flue: Kill some 30-40% of all males in Europe together with a substantial amoutn of women and children. Now that will make WWII more interesting.
White winning the Russian civil war? kicking the Tsar out (fair enough) and actually do something smart. Democracy, industrialisation, capitalism, etc. All good things
The thing that will spoil it maight be the minorities in the different countries created (and moved around) after WWI. It was not particular according to nationality. And that created some of the Unifinished business.
US doing "Vietnam" in China? direct intervention on the side of Mao/?. That should keep them busy for a while.
North Sea oil getting discovered in 1920's? And technology to somehow start drilling out there. Now that would be interesting insofar as RN was pretty dependent on Iran/Iraq for oil.
Let the oil in Nigeria be discovered in the 1920's? that will topple the chart a bit
If you keep Hitler out of it, yuo maight stil have WWII but chuck in 2-3 of these things and it willsurely be different.
PS: I like the one with the oil in Nigeria.
Ivan