Further on the breakdown idea:
After each year, move the targets needed for Disorder & Revolution down - all nations become increasingly weary of war.
So two steps to a power collapsing: Disorder, then Revolution. The latter as described above;
Disorder: the power continues to operate as normal, but combat units are liable to mutiny. Fleets may refuse to sail out of port; armies may refuse to leave the trenches to attack.
It should be possible for a power to recover from Disorder; most countries had mutinies at some point.
All powers begin on the chart at 0. It might go up to +10 for early successes, then steadily down for everyone. “Disorder” and “Revolution” markers begin at appropriate points (Revolution at the end) and are moved down at intervals, probably at the end of each year.
Perhaps another chart to track American involvement, influenced by U-Boat sinking, invasion of neutrals etc. Just possible for it to swing far enough for America to join the CPs.
Think I mentioned this before, but a way of balancing towards the CP is to allow only them to attack neutrals, and only them to annex and exploit conquered tt (the Allies have to liberate it).
On the global aspect of the war:
Japan as a power, but what about China?
Did Japan have a modern industry in 1914?
IL, do you have a global map or just sections outside Europe; you said no Central Africa.
Some other issues:
Ammunition - armies need ammo to fight. Thinking of using “bullets” from old version of Diplomacy. Produced in factories, need to be transported by rail or ship to keep units in supply. Can be stockpiled and captured.
Motor Torpedo Boats - cheap naval units which can attack bigger ships. Problem is, I don’t want people building loads of these to soak up hits. Perhaps they can only attack when accompanied by big ships, or you can choose which units to attack in naval combat?
Edit: MTBs can only operate off the coast of friendly tts.
Techs - to what extent can these be shared with your allies? I would say that, after a power gains a tech:
the next turn it can build the new units
two turns later its Allies can build it
two turns after that its enemies can produce their own version
you have to physically deliver a unit to an ally for it to gain the tech.
I’m assuming that units delivered to an ally’s home tt can be converted to its units - otherwise Turkey would have no modern weapons (it should have no factories).
Updated XL file attached.