I’m coming down in favour of a 4 player game. Since the four principal nations: were
They would be the starting element for each player, in turn order.
With other nations joining in, we should end up with:
Austria, Turkey, (Bulgaria)
France, UK, Italy, USA, (Belgium), (Portugal), (Greece), (Romania), (Arabia)
All nations within each group moving and fighting together.
One possibility is that, at a certain stage, Allies can be placed under an overall command and after that they all play together.
Ref: Supreme War Council and Unified Command
http://books.google.co.uk/books?id=vWLoU9HVBeEC&pg=PA20&lpg=PA20&dq=central+powers+unified+command&source=bl&ots=6BEE-CvHEp&sig=1ARJ-Cd3I-IAtUc93BZrS7XZO8g&hl=en&sa=X&ei=9hX7UPqjE8eZ0QWd7YGADQ&sqi=2&ved=0CF0Q6AEwBw#v=onepage&q=central powers unified command&f=false