OK, here's a provocative one, I'd have Germany win WW One ...
And go.
That isn't terribly unrealistic and it would have had a dramatic impact on history from there forward.
According to the first random internet search result that I came up with the largest economies in the world as of 1910 were:
- USA
- China
- Germany
- India
- UK
- Russia
- France
- Italy
- Japan
- Poland
- Dutch East Indies
- Spain
- Belgium
- Canada
- Argentina
This particular list, I think, is using modern borders and estimating. I say that because Poland didn't exist in 1910 (modern territory was part of Germany, Russia, and the Austro-Hungarian Empire. Similarly, India was part of the UK and the Dutch East Indies were a Dutch possession.
Also, China, India, the Dutch East Indies, and to a lesser extent Russia are listed here largely because they had really big populations. In terms of war-making potential the list is something closer to:
- USA
- Germany
- UK
- France
- Italy
- Russia
- Austria-Hungary
- Japan
#2 Germany and #7 Austria-Hungary ended up at war with all the rest. They couldn't possibly win and they didn't. However, after Britain's ambassador informed Berlin that the UK would fight for Belgian neutrality there was a last-ditch effort in Berlin to abandon the Schliefen plan (which included invading Belgium solely as a means to get to France) and invade France directly along the short Franco-German border between Luxembourg and Switzerland.
It would have been an incredible gamble for the Germans because knocking France out quickly was imperative. If they had respected Belgian neutrality and failed to knock the French out quickly their situation would have been dramatically worse than it was in the actual timeline because the British and Americans would likely have gotten involved if the war dragged on more than a few years anyway.
On the other hand, given the VAST superiority of German arms over French arms in 1914 it is entirely possible that the Germans could have knocked the French out without going through Belgium so long as the French were unassisted by the British. Additionally, while the German Fleet was no match for the Royal Navy they would have completely overwhelmed the French Fleet and could possibly have blockaded France and provided support to their army so long as the British were not involved. Better yet, without the British blockade, the Germans would have been able to continue trading with the British and the Americans.
The next obvious domino to fall is that with Britain not in the war, Italy would NEVER have joined France and if France's fall became apparent the Italians may well have joined with the Germans and Austro-Hungarians just to get Corsica and some French Mediterranean provinces.
IF (that is a big IF) the UK had stayed out then there is basically no chance that the USA would have gotten involved leaving Germany with by far the largest and most advanced economy of the combatants. France and Russia would have had little or no chance of defeating Germany and Austria-Hungary and if the Germans successfully knocked out France then Russia's fall would have just been a matter of time.
At the conclusion of this war Germany likely annexes a few French border provinces and a whole lot of Eastern territory. At that point the German economy would have been easily the second largest in the world behind only the USA and Germany was the most technologically advanced nation even before WWI.
With no Central Powers defeat in WWI there is no Versailles Treaty and thus no Nazi takeover of Germany. Hitler remains a more-or-less anonymous German veteran of WWI. Russia probably doesn't become communist because without the French and British in the West to worry about the Germans don't need to send Lennin in to destabilize Russia. Additionally, with no British Ally and with France falling, the Czar would have known that he had to end the war. What happens to Russia after that is anyone's guess.
Fascism may well have taken over in France instead of Germany but France's economy and technology would have made it much less dangerous to the rest of Europe.
My best guess is that the German Empire would have become more democratic over time. They already had a Parliament but the Kaiser was much more powerful than the British monarchs. Germany would likely have become the world's preeminent power as they had the economy and technology to do so with only the USA having a sufficiently large economy to challenge them.
The USA would be much different today without WWI and WWII.