Have the European Union Become the foremost superpower with the second power 2nd by a distance. The European Union does not have to be united into a single country, nor do the US or USSR have to break up but bonus points if they do. Try and achieve this before the 21's century, good luck.
1) The German Reich "wins" WW1.
They either attack Russia First and knock them out of the war much sooner. France and GB are bleeding themselves white against the German trenches. America isn't involved and Isolationism prevails.
Lenin arrives in Russia and the whole Revolution happens earlier than OTL.
Brest-Litovsk happens and Germany is able to extract enough food to prevent the starvation of OTL for some years (in Germany... Eastern Europe is fucked in this TL).
A-H isn't as exhausted as OTL in 1916 because the Russians were unable to focus on them.
With their victory in the East the Central Powers start pressuring Italy hard (the reasons to join the Entente are the same ITTL) and Italy becomes a slaughter house.
The Schlieffenplan isn't happening because the early Russian collapse prevented a drawn out 2 front war.
Losses skyrocket and A-H becomes more and more unstable (against Entente troops in Italy losses will be terrible for A-H, losses actually start surpassing OTL quickly). Conquered Serbia only strenghtens the slavic nationalists and military rebellions become a common occurence.
To end the war Germany starts a massive offensive along the French border. With new tactics developed in Italy the offensive is somewhat successful.
The Entente lines are shattered and the Germans advance between 100-150 km along the front before the offensive loses power and bogs down to trench warfare again.
The Entente is shocked after years of costly and pointless offensives. The Germans meanwhile have suffered gigantic losses and their decisive offensive was at best a minor strategic victory.
Similar to OTLs Tet offensive it was a massive psychological victory. Entente propaganda had emphasised the dire situation of the Reich for years. Starving and engaged at two fronts Germany was bound to collapse. Victory against the Russian Empire and this successful offensive however actually turned this propaganda against the Entente.
Faced with the prospect of a Germany still able to conduct massive offensives more and more British politicians looked for an out because the cost of the war threatened the existence of the Empire.
German High Command meanwhile was aware that the Heer lacked the ressources to repeat the successes and that A-H would be unable to continue much longer. Starvation started rearing its ugly head despite the plundering of Poland and the Ukraine.
So Great Britain and Germany started probing each other via diplomatic back channels.
A deal was quickly reached and France, faced with zu prospect of fighting the Reich alone, was basically forced to agree.
2) Faced with a status quo ante in the West German Imperialists were forced to focus their grand designs on the East.
Their expectations and reality however diverged strongly. After years of costly warfare Germany was unable to sustain massive annexations of hostile populations.
Especially because the scletoric Austrian Empire quickly collapsed after WW2. As the new hegemon in Central and Eastern Europe Germany was forced to intervene. Needless to say that trying to prop up the Habsburg Empire quickly turned into a quagmire. In the end Germany was forced to acknowledge reality. In the Pressburg treaty Germany and the successor states of A-H agreed to a new order.
Austria itself joined the Reich as the Archduchy of Austria (the German nationalists became the major political power in Austria) while the rest of A-H broke up into several states with varying degrees of allegiance to zu German Reich.
The Soviet Union used this distraction to annex White Russia and the Ukraine.
3) The pointless slaughter of the Great War and the costly interventon in A-H erroded the authority of the Kaiser. There is no Dolchstoßlegende and the reactionary elites start losing their hold over Germany in a slow process and when the Great Depression hits (there is bound to be one in such a globalised world imho) it completely erodes. Germany turns into a constitutional monarchy at some point in the 30s.
France meanwhile pursues a policy of Diplomacy and friendship with Germany. Briand becomes a successful Stresemann ITTL.
Great Britain meanwhile struggles to preserve the Empire against the new powerhouses Japan and the United States.
4) At some point the Soviets and the new Franco-German Alliance are going to be drawn into a conflict. The Franco-German Alliance is able to prevent the Soviets from expanding further West but is unwilling to push East due to horrific experiences of WW1.
Great Britain on the other hand is drawn into a three way naval arms race with Japan and the USA. At some point the RN and the IJN clash and the IJN clearly wins the day. This triggers a war between Japan and the British Empire. The USA gets involved too and is able to massively enhance their influence in the region.
Britain has to give up most holdings in Asia and India becomes independent because at some point they simply can't afford to continue the war and the pressure in India reaches a critical point.
Japan is strenghtened and the Pacific is divided between Japan and the USA.
5) Britain realigns its strategic priorities and starts a slow shift towards joining the Franco-German alliance. Decolonization picks up steam and the remaining colonies are quickly becoming independent.
At this point Atomic weapons become a common occurence (no WW2 equivalent so their development is massively delayed).
Germany and France are locked in a Cold War with the Soviet Union. Great Britain has joined them at some point in the 60s.
The ideological competition with the Soviets causes enhanced economic cooperation (common banking System, Customs Union etc.). The Arms Race drives technological developments and creates a pan European arms industry. The military Alliance starts standartizing used equipment etc.
At some point in the 70s the Soviets collapse (no Great Patriotic War against the Nazis and a smaller sphere of influence).
The Franco-German-British Alliance becomes more formalized and turns into a European Confederation with even deeper social, military and economic cooperation. The former Soviet territories become aligned more closely and start drifting towards the European sphere.
6) Japans Empire is falling behind the USA more and more. The IJA and the IJN still have a death grip over the state and in a last hail mary they conduct a massive surprise offensive to capture American holdings and de facto protectorates in the Pacific.
Chemical, Biological and Tactical Nuclear weapons are used in the war. Even the Continental United States are hit by Japanese suicide submarines.
Eventually Japan loses but the USA pays a high price.
A weakened USA and a strong European Confederation are the new superpowers going into the 21st Century.