Surprised how everyone thinks that killing individuals (even if they were influential individuals) could stop a world war. It was always coming, with international tension etc. etc. especially in the FE. I'm all for keeping individuals alive.
Anyway to go with the mood: Kill Otto von Bismarck in 1867 = no Germany = balance of power maintained in Europe (at least for a couple decades). The Franco-Prussian war may still have happened, but the result would have been more even.
Seriously, though:
If I had the power, I'd convince Frederick III not to take up smoking. When he assumes the throne 1888, he's not already half-dead from throat cancer (or something like that). As per his wishes, the German Empire becomes the German Commonwealth (i.e. constitutional monarchy with a federal government similar to modern Germany's system, each state having their own constitution, all under the purview of an expanded Zollverein), Bismarck is sacked if he starts any trouble. Reconstitute the Grand Duchy of Warsaw (to appease the Poles and to piss off the Russians), grant relative autonomy to the Duchies of Elsass-Lothringen (to piss off the French... I will forever rant that what France did to them was waaaaaaaaaaaaay worse than what the Germans did), cede Lower Schleswig to the Danes as a gesture of good will to them (nigh worthless anyway), but demand the Heligoland in return (using such diplomatic language as being 'defender of the Baltic').
Politically isolate France and Austro-Hungary (Bismarck was doing the former anyway well into the 1880's). Confirm alliances with Spain (purely for show, but ensures France will be surrounded by enemies if they play the aggressor), Denmark (and help them rebuild their navy, in case Russia starts anything) & Britain (no-brainer, since direct in-laws in power).
This then means that Germany controls land trade in Europe, while Britain controls sea trade (effectively). The Russian Revolution doesn't occur (unless Nicholas II is dumb enough to invade Poland) as the Germans have no reason to shelter Lenin. Even if Archduke FF is assassinated, the Germans won't intercede on their behalf (there's no benefit to them whatsoever), so Austrians vs. Serbs = shit army vs small army, stalemate after a year, guerrilla fighting for another two, the rest of Europe mediates after and partitions the Austro-Hungarian Empire, giving Serbia it's own autonomy, Hungary likewise gains independence along with Romania, leaving Germany to boss it in central Europe.
Therefore, major war will be Russia vs Germany: Britain sides with Germany (family and bad blood from Crimean War), France sides with Russia (dislike of... well everyone else). Japan sides with Germany for their own interests, but with the RN and KM in the pacific, they are curtailed in their expansion south for political (as opposed to military) reasons. Danes and Germans bottle up the Baltic straits, and we all know what happens when a Japanese fleet and Russian fleet duke it out. There's no stake in it for the Americans (and as they are still pursuing isolationist policy), they do nothing. The Ottomans start their own war by invading the Balkans again. Russia's armies are tied up in northern Europe, so Turkey and Austria start giving each other sneers again. Eventual result: draw, Treaty of St. Petersburg: Russia cedes some of its eastern territories to Japan, grants independence to Ukraine and Crimea, Poland (including that originally controlled by Russia) arises as a protectorate of Germany (with own government, but under suzerainty from Brandenburg), north east China is occupied by a mixture of European militaries after much wrangling to try squeezing out the Japanese (personally, I'd rather have seen a partition, but never mind).
Next war: Ottomans vs. Austria. Germany comes to the latter's rescue, absorbs them into the Zollverein.
Can't be bothered to conjecture further.
And we're back where we were in 1710! ... sort of. Except that Prussia/Bavaria/Baden-Wurttemburg etc. = Germany. And Erich von Lewinski is German Chief of Staff in the 1930's!! LOL