I have a problem with the fact that the rate of manpower gain doesn't change at all during the war. Granted, if a country loses territory this goes down, but the fact that manpower was diminished but frontlines hardly shifted on the western front is something that must be addressed.
After 4 years of war, isn't it odd that I have no manpower problems at all, assuming that I havent gained or lost much territory? If I'm low on manpower I just have to sit around and wait until it goes up again.
I therefore propose that an event fires in mid 1916, that reduces the manpower of major cities of Britain, France, Russia, Germany, AH, Italy. Another one fires in mid 1918.
The counterargument is "How do you know they undertook massive offensives?"
However, I think a prolonged war will in most cases lead to heavy losses, and this should be reflected on the rate of manpower gain per month.