Like... I already gave the idea for having the Entente be: Me, Britain, France, Italy, and the Dominions. The Central Powers could be: The Union, Germany, Austria-Hungary, Ottomans, Russia, and Japan. That would be a much fairer world war, it's just that half of the Central Powers have no active player.
All we need to do is figure out who goes where and make sure those nations have players. The Aztecs can maybe go the Central Powers and Japan can be with the Entente instead. Make a definitive list of the sides when ww1 roles around, then we can figure out how to fill those major power slots in the years going up to ww1.
WW1 would consist of the Union, Spain, and the Aztecs getting curb stomped by literally every other active player. the war isn't so great when one side just murders the other in a week.
Yeah, like I just said. we already have a broad idea of who goes where, lets just take all of the major powers and reshuffle them between the two factions to make it balanced, then try to find players for the empty nations. Britain, France, Me, and Italy are definitely Entente, but other powers like North Mexico and Japan can join. Germany, Austria-Hungary and the Union are definitely Central Powers, but Russia, the Ottomans and China can join to balance it out against the Entente.