First of all, I think congratulations are in order for an interesting game. I’ve played 2,5 times WWI Gold (3,5 because my hard drive had to be replace and I had to start the game from turn 1).
Briefly: I played 1918 scenario once, which ended in CP victory before the end of 1918. Germany is still quite strong, but was probably crucial was my success at starting a US-Mexican was which deprived the Entente from some much needed reinforcements.
1914: I played it once as CP and once as Entente (both decisive victories). CP won by mid 1917. France had collapsed in mid 1915, because it had a hard time stabilizing the front in the north, when I managed to get Italy on my side. France has practically no chance to fight a war in Belgium, France and Italy. Russia somehow managed to survive multiple revolutions and surrendered only when it was left with no army (there were only a few scattered units, but none of them above the middle of the map). By that time I had invaded Cyprus, Malta, Libya (lost in 1915 and retaken in 1917) and wad planning the invasion of Britain (unfortunately the war ended one turn before the planned invasion begun).
The Entente game was surprising easy. By end 194 I still controlled much of Belgium (the excellent British units had retaken Brussels), I had isolated and destroyed a small German breakthrough in the south, had seriously damaged the AH army, and managed to get one of my roaming Russian cavalry units t conquer the undefended Berlin, thus ending the war!
My conclusions are that the 2 most important and underestimated aspects is cavalry (especially in the east) and diplomacy. If you use the aggressively (which the AI does not do) you could win the game the game easily. Always invest as much as possible in diplomats, and try to get countries to join your cause. The economic assistance of the Scandinavian countries is useful, but Italy (for CP), Romania, Bulgaria, Turkey, but also Spain (Entente garrisons), Iran (can invade India or Turkey), China or Japan are more useful most of the time and I was able to get most of them on my side in most games. Especially if you are doing well militarily the momentum could allow you get the world (almost) on your side.
As for cavalry (both sides) using it to get behind enemy lines and disrupt his railways, conquer cities and isolate units can win the game. You might lose a few of them, but the enemy will lose more units to isolation.
Now for the problems/suggestions. 1) I think something has to be done for the Entente invades Belgium event. In the game I never finished I managed to get to the Somme unopposed by playing this as CP. In reality of course no country would invade another with no army as Entente seemed to be doing.
2) Japan. If you get it on the side of the CP you could use is fleet against the British, even though it is supposed to be fighting a war only against Russia.
3) I noticed a couple of times that my navies (both as CP and Entente) appear to have fought a brief fight against the enemy (noticed damaged fleets on both sides) even though I had never seen naval combat. The fleets were not in raid missions or in (they were just outside) enemy minefields.
4) There is a problem sometimes with reinforcements. If you try to get a unit to join an army that's full, it sometimes joins the next, or the previous one, which can be at the other end of the map. Something like that can ruin your game.
5) I think The Averoff unit of the Greek navy should only have one ship. Elli (the ship I think that's in the Averoff unit) was a light cruiser of a deferent class. The Greek navy had ordered one BC in Germany (guns from the US) which never arrived. You create an event for this. Also Midili in the Ottoman navy, is Breslau, and be following the fate of Goeben and not be included in the Ottoman navy in the beginning.
6) If the game goes on to 1919, maybe you could have the possibility of aircraft attacking ships. The first major ship was sunk in tests in 1921 (thus wwi technology more or less). You could argue the doctrine did not exist, but if the technology is there the doctrine could change, especially if the circumstances are different from the historical wwi (for example CP invasion of Britain). Related is the issue of the technology tree. If you play the game long enough and invest heavily in technology, you usually discover everything by 1916/1917. Maybe some more technologies should be added, or discovering them could become a bit slower.
7) I sometimes could not get the navies to go where I wanted. I had the Italian BCs at Naples and wanted to get them in the Med, but it was impossible. The only way I could do it was to get them to leave to port in the first turn, then get them off the coast of Sicily, turn 3 to Taranto and then to the Med. Also the Br Atlantic fleet takes 2 turns to get to the open seas (at least I could not do it in one).
8) Greece can join the CP too easily. In reality joining them was never an issue. Even the pro CP side, was only for a pro CP neutrality. Under different circumstances (defeat of the Entente navy that threatened Greece’s long & vulnerable coastline?) that could have changed.
These are my thoughts for now. Sorry if the post is too long.