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The current plan is that there are 4 established power blocks:
The US+Canada, China, Russia, and what remains of the United Nations (which equates to 'everything not claimed by the above three').
These represent the big four contenders within the UN during the Angelwar, and aside from the North American Alliance, China and Russia there isn't really anywhere that could hope to oppose the UN on their own.
We want each power block to have a shtick or two, but we are still pretty up in the air as to what that would be.
Russia is the 'generalist' army, supplemented by the fact that Post-Impact they were the only ones with a functional space program, so expect Russian orbital masers.
The UN, as the original Eva developers, has all of the really high tech. Seele is probably still around, but obviously weaker. Expect MPs.
China built nearly all of the Evangelion scale weaponry for Nerv back in the day, even if they didn't get their hands on actual Eva tech until the very end. Expect sub-par Evas arms with firepower rivaled only by the UN and conventional forces armed with positron tanks.
America is just enjoying their wartime economy and chugging out everything. Their Evas are probably the lowest tech, relying on BULLET HELL and MISSILES EVERYWHERE to see them through.
You 'defeat' an enemy in WWEva by destroying their Evangelions/Eva-scale threats, destroying their Eva-scale production facilities, halting their research projects, and taking their capital. Even if they technically have an active army after this, they are incapable of winning after this are can be safely ignored by the Evangelion-piloting players as they are deployed elsewhere against more important targets.
Possible missions include hunting down enemy Evas, assaulting enemy bases, and terror missions to run rampant in enemy cities and cause as much collateral as possible to ruin enemy morale.