Aftermath
The question is how this TL will develop further. To give you a start:
"Mitteleuropa"
The customs union and military alliance will probably continue to expand under German leadership and eventually lead to a kind of European Union in the long run. Will Austria-Hungary, Romania and Bulgaria join after some years? Probably also Sweden, Denmark and Greece? But does this also apply to the countries in Western Europe? How do France and Britain react to this development?
Poland
In fact, like the other new states, this will become a German puppet state. The Polish minority in Germany, mainly living in Posen and West Prussia, will be "encouraged" to settle in the eastern Polish areas of Wolhynia and Polesia. What will happen to the Ukrainians and Belarusians living there?
United Baltic Duchy
This protectorate is in fact ruled by a German-speaking elite. New German immigrants are likely to be attracted to strengthen this elite. Will they be able to maintain power? Or will this state eventually disintegrate into Estonia and Latvia?
Austria-Hungary
Will Austria-Hungary survive? The status of Bosnia, Serbia and Montenegro is likely to cause problems. What will be the solution?
Trialism? In which all South Slavs will be included in one new kingdom to be formed? Or will this be an intermediate step towards the independence of Hungary and “Yugoslavia”? Under that circumstance, will Austria join Germany?
Russia
The war ends with a major defeat for Russia. Will there still be a revolution? Will the Tsar have to resign? Or will he only lose his power? The Bolsheviks are unlikely to come to power. How will Russia develop?
Ottoman Empire
The war has brought nothing to the Ottoman Empire, except that the Northeast has become more ethnically homogeneous. The Christians around Alexandretta are likely to flee to France for fear of reprisals. Relations with Germany will have cooled because Germany has not advocated annexations in the Caucasus during negotiations with Russia. Will the Turks be able to continue to dominate Arabia? Or will the Arabs revolt sooner or later? Will France intervene again “to protect the Christians”?
May I give a shot in a possible future?
Mitteleuropa
Mitteleuropa with Poland, United Baltic Duchy will initially start as a custom union, beneficial to German companies, and at the start a union were German politics can force its political power. Although the political influence will decline within a decade as the result of the economic success of the custom union. The economies of all countries will experience an unprecedented economic growth. With this economic growth a self conscious will arise and simultaneously a confident resistance of local political leaders and parties towards too much German political interference.
Germany it self will be over confident due to the very successful result of the war, despite the enormous number of deaths' and mutilated men. German population will be overly pride first of the war result and later due to the economic boom it will experience. However the economic growth and wellfare of common Germans will put pressure on the political system. This will gradually give room to a more democratic society were the Junkers need to give room to the common German in matters how the country is ruled. Initially this will find resistance of the old elite and military leaders but in the end the old guard will lose it of time. Germany was before the war already a sort of proto democracy, which declined in OTL ww1 in to a militarily junta. At the end this will be beneficial for Germany since in the first decade after the war German , many politicians and military leaders think that armed conflicts will solve any foreign problem. ( as they thought after the Franco-Prussian war). The huge success of the army and the rather secondary role of the navy mean the slow and long end of the capital ships of the German Imperial Navy. Capital ships will be replaced for modern versions during the next decades but the battle fleet will not grow any more in size, rather shrink in favor of much cheaper units like cruisers, destroyers and submarines, which are even better suited to protect trade lanes.
Austria Hungary.
After a few years of calm the unrest will start. Hungary and Bohemia will demand more influence as it did before the War. Other Nations within the empire are demanding more independence or are afraid to be over powered by the Hungarians. The Austrian Empire will experience a period of social unrest in some parts of the Empire being a state of civil war, in all but name. In an effort to keep the Empire together it will split in very autonomous countries like Hungary, Bohemia, Austria and Slavic nations. Forming an Empire only in name, as a kind of commonwealth. Or it will fragmented in independent nations were some of them will join the custom union of Mitteleuropa. This will happen during the course of the 20ties until the start of the 30ties
Russia,
Will experience a decade of unrest and some sort of decline, but will recover in a stable, albeit light repressive constitutional monarchy ruled by a conservative but social authoritarian leader. The country is democratic but on a Russian way, were industrial Oligarchs and Nobility find each other and keep the masses happy with an improved economic situation and living conditions. In the short decade of unrest, no doubt the radical Bolshevist will try a shot to power but will fail. Initial they are successful but will fail at the end, both due to their radical nature. This will last from 1916 up to approx. 1922-23
After this decade of unrest the Russian Empire will try again to assure it's place on the world stage. Possible they focus their attention on the Ottoman Empire. Trying to grab apart of Ottomans which they conquered during the Great War or even more, since Oil become by this time a highly valued commodity, which you can not have enough.
This they will do on their onw or in conjunction with other powers. Not necessary in a combined effort.
Ottoman Empire. The ethic cleansing of the former Armenian territories will have a long lasting economic negative effect on the Empire. The Ottoman Empire fail to modernize it self, despite efforts and even progress is made it will be nullified by the progress in Europe.
The unrest in the Austrian Hungarian Empire and the poor performance of the Ottoman army in the war, could encourage Greece and Bulgaria to start a war against the Ottoman Empire. Or the Ottoman Empire will start a war against the Greece in an attempt to show their power. This can happen 1920 or earlier or perhaps as OTL 1922. One way or the other the Greece will be backed by Great Brittan, and possible Russia and France.
Many countries will be eager to tore the Ottoman Empire apart all for various reasons. Publicly to protect Christians or Greeks but in case of France and Great Brittan to gain the oil fields. Oil will be by this time a the most desired commodity. By 1930 the Ottoman Empire will will just a rump state after a long series of very bloody conflicts. In Great Brittan there will be factions who are more than eager to go to war with a weakened opponent like the Ottoman Empire since these faction idea is that hey missed a war with all its opportunities.
Colonialism will see its zenith in the next 20 to 30 years, both in Africa and Asia. Germany will pore money in their now vast African holdings. Albeit as pre war Kameroen it will be for years show a negative balance sheet. This show, as with many other African holdings by European powers that these possessions are more a burden and only held for prestige, of course there are exceptions.