I know these academic exercises are popular for WW II (what if Germany wins the Battle of Britain....or defeats Russia, etc) but what about The Great War?
How different would Europe & the world have been if the Central Powers won? Say Imperial Germany captured Paris in the opening weeks of the war (instead of out-stripping their supply lines and 'losing' the battle of the Marne) or the US didn't enter the war? Or whatever not-too-far-fetched scenario leads to a different result. Would communist revolutions have replaced the Fascists? How long would Austro-Hungary have held together? What would the German Empire look like in Africa? How differently would the US have developed & looked at the world?
IMHO, this is a very tough question to answer, because for Germany to have gotten everything it wanted at the time - to be the pre-eminent global power - it would have had to have completely finished off Great Britain's naval power. Germany probably could have won against France and Russia, but Great Britain would have been one tough nut to crack. So, Germany's best bet in 1914 would have been to keep the British out of the war.
If Germany could have kept Great Britain neutral by absolutely safeguarding the neutrality of Belgium, they would have had a major chance to change the course of history, assuming they could have finished off the French through the truncated border (I personally think they could). They could have thus picked France and Russia off, saving Great Britain for later. One wonders, though, if Great Britain would have remained neutral even if Belgian neutrality was respected. Britain had been, for the prior few decades, playing nice with its erstwhile foes France and Russia precisely because of the growing threat of Germany.
Even if Great Britain remained neutral for that first war, it wouldn't necessarily have solved the problem. Again, Germany (or maybe it would be better to say Wilhelm II) didn't give the impression of just wanting continental hegemony; they wanted a global empire. And again, any German global empire that could have existed would have existed under the constant threat of the Royal Navy. Eventually the two would either have gone head-to-head, or one of them would have had to have given the whole rivalry up. Great Britain wouldn't have, given its history up to that time.
Personally I think the odds are against a peaceful if uneasy co-existence. Germany would have been flush with victory against France and Russia and would probably have ended up at loggerheads with the British over some colonial issue. Then there would have been another global war. So you now have an alternate "World War II," but one in which Germany and Great Britain would have more relative power - Great Britain would not have exhausted itself in the first war; but Germany would have a much more global reach and easy access to Middle Eastern oil via the Ottomans.
If, on the other hand, Germany could have moderated its aims and accepted second place to Great Britain, things look very different. That might not be a huge stretch, either - one wonders if a shrewd Germany could have diverted the British into a new round of the Great Game against a weakened Russia, and in the meantime consolidated their hold on formerly French African and Asian possessions and spheres of influence.
The latter could give rise to interesting possibilities in Asia - Japan, taking the part of the 'losing' side in the first war, might end up in a shooting war against Germans in China. Who knows where that might have led.
As far as the Balkans, after the defeat of Russia in the first war, the 'powderkeg of Europe' would probably have gotten a much longer fuse. It would have continued to be a source of problems, but Austria-Hungary and the Ottomans might have had a grand old time trying to sweep up the pieces. Austria-Hungary would probably have gone ahead to become somewhat similar to the current British Commonwealth - the various nations (Austria, Hungary, Czechs, etc) becoming separate, but remaining friendly with the same monarchical head of state. The Ottomans might have tottered on for quite awhile, supported by the Germans to facilitate oil flow.
Finally, the US would probably be quietly growing into an economic superpower, but would probably have remained fairly isolated, especially if Japan's gaze was turned towards China.
Anyway, those are just some thoughts.
Good thoughts... In retrospect I guess I was really asking 2 questions: How could the Central Powers have plausibly won? and then what would have happened? Obviously, the first question is much easier to imagine then the second. Like you, I find it hard to see the UK staying out since they were so concerned about Germany and the defense of the Empire.
So without going to wild speculation what about this scenario for German victory? The Austrians pull their weight on the Eastern Front (like Germany wanted) and commit to Russia first instead of attacking Serbia. German troops/supplies are freed for the initial push into France and captured Paris (as they very nearly did in reality) and effectively remove France from the war. Then the Ottomans actually capture the Suez (instead of the one weak effort they gave) and been even slightly more successful at fomenting Jihad in UK's middle Eastern/African colonies threatening India. The first would have allowed Germany to commit immense resources on Britain alone on the Western front and the second would have seriously diverted UK's attention to her Empire, which from all I've read is really all she cared about anyway. This would preclude the need for unrestricted U-boat warfare and the tensions with the US.
What happens then? That is tough to say....it's almost impossible to imagine Austria-Hungary not disintegrating. I think Germany even had a contingency plan to annex large chunks of the Teutonic populations (Imperial Anschluss!) I also think the Russian Revolution was nearly inevitable after the war started. Still leaves the US & Japan on a collision course in the Pacific and an Imperial Germany with hegemony in Europe eventually would conflict with the Soviets (even weaker under Brest-Litovsk) and Great Britain. If the Kaiser would have eventually attacked Communist Russia would the US & UK been willing to come to her aid? If not then Germany would have been a real beast with no continental rival to deal with and could spend resources building a navy. I'm sure that is overly simplistic but interesting to contemplate.