right now I'm thinking of having six major combatants with special tech trees (uk, france, germany, russia, austria-hungary, ottomans) and two additional fence sitters (spain, italy) with other goodies
nations outside of europe will be playable, however, their roles will be to organize overseas supply lines and whatnot to their allies on the frontlines and conduct underground operations rather than put boots on the ground in europe. after a certain point when naval transportation has been improved enough, they may sail troops over as well.
any nation outside of the initial six/eight will not receive special tech trees (theirs will be generic "infantry," "armor," etc.)