Hmm. If I had my druthers, I'd suggest alternating between the USMC and US Army once the first four factions have been "fleshed out" acceptably - after all, in theory, you can just keep putting out new adjuncts to the EFTF (There's Spain, Italy, Switzerland, Denmark, Sweden, Norway...uh...Ireland...). It strikes me that the relationship between the Army and Marines, in terms of how it all works out on the tabletop, is not disimilar. Of course, that leaves the question of where to go in such a manner with the MEA and PLA...
Nah, if I had my druthers, like I was saying, I'd consider Japan for the second wave. After all, they have a pretty well trained and equipped force, and in the wake of Chinese belligerence - that is to say, in heated disputes between the nations over resources - they HAVE been considering nixing that line in their constitution preventing them from starting a war. Banzai!