Let's say, in 1918, rather than surrender, the Germans somehow kept the fight going. What would happen if the Allies took the war onto German soil?
Maybe there could have been a WW1 Berlin battle. As allies pushing forward, Berlin is encircled by French,Belgian, British Empire , Commonwealth , Italian (coming from the South, already pushed through Bavaria ) and American troops , maybe some Czech Legion volunteers and others. Exhausted Heer units and inexperienced and ill-equipped Landwehr units try to hold the Entente advance but fail, some Heer troops from the east and the Baltikum, previously fighting Bolshevik troops, hasted back to Berlin to defend the Reich´s capital.
The Kaiser was suggested to die as a propaganda martyr figure on the front to save the monarchy(some German princes follow this Prussian example), he did and the crown prince is pronounced Emperor but there is no coronation, as Potsdam is already under Allied command. Meanwhile Ludendorff and Hindenburg and the OHL reign as a quasi military dictatorship as the German Empire begin to dissolve. In an desperate atempt to save the sitution, in a secret note Ludendorff offers the Entente a Alliance against the Bolsheviks in the East, the offer is immediatley slammed.
The German propaganda blames now Social Democrats and Bolsheviks for the failure of the Heer. There are heavy reprisals against opposition , far-right paramilitary groups like "Stahlhelm" crack down Opposition, there are Anti-Semitic slurs. Ludendorff orders the Reichstag to dissolve. The Berlin population, weakened by starvation and sickness, is not ready to fight a battle but they are ordered to fight along the Heer anyway.