First I doubt it will get a "World War" moniker. The best scenario would be revanche Germany and revanche Russia eventually running into each other over Poland. You may have seen an Anglo-Japanese War or a Pacific War (with USA added or substituted) but it plays out apart in time or connection. If you want to get strange have Weimar Germany and Republican Russia grow close and then get revanche on everyone. That feels more fictional to me but could become a global war as both have axes to grind everywhere. And I am guessing it is a worsening Depression, inept everyone else, and many butterflies that build that ATL.
Unless one intends that an Entente "win" leaves Imperial Germany less broken and Russia more fully like Weimar Germany, a leftist Republic embattled by the right-wing anger fest. That gets you far more traction for another war on global scale. It could be revanche Germany or simply bat shit hate orgy Russia under a maniac demigod, either has reasons to start a war for the glory, for the money, and for the show. Or it could be more organic as the Empire(s) break down, bush wars and proxy fights get out of hand as independence for one is revolution to another, world at war.