There is a strong argument that a WW1 Scenario should begin either on the day of the assassination or not long before it: the Germany of 1914 was not engaged in the kind of high-speed rearmament that the Germany of 1936-39 carried out. Germany already had a large army and navy in 1914 and probably could not have gotten much stronger with reasonable peacetime expenditure. Having the player start earlier than 1914 simply allows them to build up an a-historically massive war-machine.
.
I second that, a- 1st January or even 1st June 1914 start is the best. January will allow 6 months of build-up, nothing too great whereas June will just allow the player to position troops on the Border and mobilise instead of waiting in the interiors with un-mobilised troops.
1st June 1914 seems the best considering the circumstances. Day of Assassination is a bit too short, considering the "Mobilisation times in WW1 cannot be easily showcased".
On the other hand, Germany did have a lot of scope to expand their war-machine had the "Political will existed". The German Fleet was as some English politicians wrote - a "Luxury Fleet". It was too big and thus became a threat to England but too small to actually threaten the Allies (UK+France+Russia were some 2.5X times stronger than the Germans, UK alone was nearly 2X stronger though in performance terms they were about 1.5X stronger).
On the other hand, Germany was spending just 3.35% of GDP on the military, UK was doing only 3% but France did over 4% and Russia did over 6%, Austria did just 3.2%-3.3% as did Italy. And these figures are post the 1912 expansion in armaments by Heeringen and Falkenhyn.
Again in terms of recruitment they did only 52% of population, France was doing 83% though Austria did just 39% and Russia only 30%.
A part of the Fleet if was not constructed, say- the BAYERN CLASS BBs and their support ships, money equivalent to 2 Corps could have been found easily, an additional 2 corps on the Western Front (and assuming no panic by von Moltke Jr) means the Western Front could have been crunched in 1914 itself.
P.S: All this is Hindsight 20/20.