Okay interesting. So in response then I would ask what the chances of a scenario like the following occuring. Germany, realizing that due to war weariness and the sheer size of the lands obtained through Brest-Litovsk makes them impossible to hold, establishes an "Empire of the Ukraine" containining the lands taken from Russia, including the Crimea and the Ukraine itself. As Tsar Nicholas is dead, they invite his brother in law, Alexander Mikhailovich, to be Tsar of the Ukraine. As for the Baltic States, they are established as German client states, and Poland itself is annexed entirely. Also, Wilhelm steps down and his son becomes Kaiser Wilhelm III. Outside of Europe Germany is able to keep Kamerun. Is this scenario plausible? and what effects would this have on the world at large? For instance would the Soviet Union be a possibility, as the Empire of Ukraine could be a home for discontented Russians, and a staging base for the White Army.
Situation in the East:
Well, the treaty of Brest-Litovsk in OTL recognized Finland, the Ukraine and Georgia as independent states.
According to what I remember, Germany had hopes for a German King in Finland. Although his role would probably be more like the Swedish King. Restricted in a constitutional monarchy.
The Ukraine including the Crimea. Independent from Russia but I can´t see a Romanov as King.
More probable is either a local noble, an Austrian or Bulgarian (Orthodox faith like the Russians) one. I´d guess an Austrian one converting?
Georgia probably will be mostly left alone for now. Especially as access is only possible from the Ottoman Empire. Later on, in case of a civil war, Baku might be tempting....
The Baltic states. Probably unified into a Baltic Duchy with a German noble as Duke and Head of State.
Poland. There is no way that Germany will annex Poland! It would simply add to many Poles to Germany. An independent Kingdom of Poland (maybe slightly enlarged with parts of Belarus) sounds more likely. With a German or Austrian Catholic noble as King.
(I believe in OTL Austria-Hungary pushed for that solution.)
Now I wrote "independent states". Nominally they will be independent of course. But with defense and trade treaties tying them to Germany. With the threat of a still existing Russia in the East, the Germans won´t even need to resort to pressure themselves.
Situation in the West:
The situation here is a lot more complicated. Even if the Kaiserschlacht was totally successful here, nothing can be done about the Royal Navy.
Likewise the British in early 1918 are occupying large parts of the Ottoman Empire (Germany pre-war was interested in oil exploration in the Kirkuk area).
And we haven´t even mentioned Austria-Hungary, Serbia, Bulgaria, Romania and Italy yet.

Or Japan in Asia.
So any peace treaty here will be a compromise.
The British (and Americans) will insist on an independent and whole Belgium. And they´ll try to limit the damage to France as much as possible.
And given that Germany is exhausted --> compromise peace.
German colonies occupied by Japan are lost. No way to get them back.
Likewise it would be awkward for Britain to tell South Africa (German South West Africa) and Australia / New Zealand (German Pacific possessions not occupied by Japan) to please give all back. Then there is German East Africa. Needed by the British for their Cairo - Cape Town railway but German colonial troops under Lettow-Vorbeck are still fighting. Making things difficult for Germany too.
German Cameroon (occupied by France) and Togo (divided between France and Britain) on the other hand are easy to give back to Germany.
So you´ll have a lot of possibilities here.
Like for example:
- Independent Belgium with its colony of Congo as a concession from Germany and in return the Ottoman Empire keeps Lebanon, Syria and Northern Iraq for example.
- The Germans not getting back all their pre-war colonies --> Germany gets back Togo and Cameroon plus some former French colonies as compensation. Maybe throw in some Pacific islands.
- Japan pays a symbolic low price for the occupied German colonies, Germany pays a symbolic compensation to Belgium.
- France gets to keep its Briey-Longwy iron ore mines and Luxembourg becomes a member of the German Empire.
- De-militarized zones in the French (raze the fortress of Verdun and Belfort) border regions against ...?
- And as I said above, you´ll still get to play with Austria-Hungary, the Balkans, Italy...
Domestic situation in Germany:
I simply can´t see Wilhelm II abdicating right now. It might happen once the political situation in Germany becomes "difficult" which will happen very soon after the war.
Quiet frankly, by 1917/18 the Emperor was seen as a joke. Rear echelon battlefield tourist not doing anything. Now politically there will be a push in Germany for a more constitutional monarchy, limiting the political powers of the Emperor. It´s possible that Wilhelm II might abdicate in a temper tantrum then when he discovers that no one is willing to defend his "God given right to rule". So Wilhelm III is a definite possibility then.
Effects on the world at large:
The Germans will do a 180 degree turn-around and support anyone who fights against the Bolsheviks. And with WW1 ended they´ll have lots of surplus military equipment. Plus military advisers.
I could see France and Italy becoming instable after a lost WW1.
Having sacrificed so much for no reward...
The Great Depression - if it happens - would also be very different.
In OTL Weimar Germany needed (short-term) American loans to pay reparations to Britain and France. Who then used the reparations to pay back their war loans from the USA. Simplified version.
So the Wall Street crash in 1929 immediately affected Europe when American banks called in their short term loans and refused to give out new loans.
Here Germany will face a period of higher inflation after the war since Imperial Germany will have to pay back their domestic war loans. On the other hand we´ll have four war years of suppressed domestic consumption so higher tax returns might pay for some of the war loans.
Most importantly though the middle class in Germany won´t be impoverished.
And another thing.
A victorious Germany won´t loose its foreign investments (properties, subsidiaries, patents, trademarks) in Entente countries and neutrals. That should help the German economy quite a bit.
So any Wall Street crash probably will affect Germany less than in OTL.
The Entente (Britain and France) will have to pay back their war loans to the USA. Up to 1917 these loans were secured with Entente property.
In OTL after the American DoW new loans were guaranteed by the US government and no longer needed collateral. I seem to remember that by late 1917 even Britain had run out of available collateral?
Additionally Britain made war loans to France, Belgium, Russia, Serbia and later on Italy. I suspect Britain might be a bit worried about these loans now?