My desire is to be able to extend the length of the First World War for two reasons.
First, it will allow me to field the interesting Liberty tank and Char 2C, neither of which were fielded. I just like them, what more can I say.
Secondly, it will allow me slide into an alternate British Civil war scenario, based on the unrest and strikes that were part of the unrest that resulted from the effects on Britain during the war years. So I talked to my uncle-in-law about this to get some solid ideas.
As I understand it the Ludendorff Offensive failed due to the poor logistical follow-up due to a lack of transport and tanks that prevented the Germans from obtaining their strategic goal. This offensive is discussed in Julian Thompson's book Lifeblood of War: Logistics in Armed Conflict, pp.46/47. Also at the tactical level the Germans had little or no armour, and no battlefield radios, so having initially succeeded they were unable to maintain momentum. All the Allies had to do was keep their nerve until the German offensive ran out of steam.
Therefore this operation by the Germans seems to me to be the key thing that needs changing in my alternative WW1 history. The strategic goal, to drive a wedge between the French and British Armies in France, and thereby forcing the British back on to the Channel ports (for effectively a 1918 version of Dunkirk), was clearly a key to any German victory the reality was the best they could do was negotiate peace terms.
Ironically, the unforeseen consequence of the Ludendorff Offensive was to ‘unlock’ the three and a half years of stalemate on the Western Front, and therefore restore mobility to the battlefield again. The Allies were able to take advantage of this new situation due to superior logistics and the new combined arms evolution from fielding tanks. This is seen quite clearly at Amiens in August 1918, when the British Armies under Haig inflicted the "Black Day" on the German Army, which the Germans were not able to recover from. For more read Why the Germans lose at War, by Kenneth Macksey. See pp. 52/53. Summary; the German military have brilliant tacticians, and are outstanding at the operational level, most of the time, but are lousy strategists.
Therefore the Germans best chance of prolonging the war was to have:
1. Not embarked on a policy of unrestricted U-boat warfare, which brought the USA in on the side of the Allies, but I want Americans so I need to leave this in.
2. Stayed on the defensive on the Western Front behind their hugely strong positions, which means no Ludendorff Offensive. I can use that.
3. Instead offer to make real concessions in the interests of peace. For example dropping one of their conditions that keep the French territory they had overrun at the beginning of the War. Easily enough assumed to have occurred.