Well, a simple way to choose the most important factions in a military point of view would be to to pick the ones with nuclear weapons : not that I would like seeing them in the game, that would be to much work for less fun, but that represents the military development. Now these are :
Russia, USA, UK, France, China, Pakistan, India, Israel, North Korea.
If you want to do a "World War" these must be in, as they are also some of the most important military powers.
Other factions could be Turkey, Iran, Germany, Canada... As there must be an arab nation, why not Saudi Arabia ? Or some sort of north-African confederation ?
Also, I don't know why you want Germany in so much, forgetting about major military powers as France or Israel... Germany has not even a professional army, they are conscripts since 1945.
PS: first post