Personally, I really hope they will add as much variety as possible and that it would be pretty interesting things and units indeed. I mean, there is actually indeed shitton of countries that took part in WW1, some of which are not known at all or known only a little (like Portugal, Romania, Siam and many others) when it comes to WW1, not to mention various special units, forces etc like Russian and Siamese expeditionary forces and so on. Or things like siege of Osoweic Fortress also known as Attack Of The Dead, with Russian Imperial Troops charging at Germans while coughing blood and chunks of their own lungs out, Eastern Rising (though yes, I know it's not a part of WW1 itself), East African campaigns, and a lot of other stuff. Well, not gonna write a whole novel, I guess you know what I mean. However, knowing EA and DICE these days, I wouldn't be so excited, cause they are known to give beautiful trailers and shitload of promises, while final result are not so awesome and oftenly are even disappointing, I mean it is kinda strange that for example there is no sign of France in the trailer, even though France played one of the huge roles in WW1, but well... I think they will introduce them later... maybe. But anyway, I REALLY HOPE they won't screw this one up, plus it would be extremely disappointing if all their talks about "more diversity" will just end at or even turn out to be just excuses to push a black guy (Harlem Hellfighter, if he is one of playable SP characters of course) and female (Bedouin Warrior seen in the trailer, they confirmed she is playable in SP) as playable characters (not that I got anything against that though) just for the sake of it (like in terms of political correctness), while game itself will actually have not so much to offer in terms of variety.
P.S. Sorry for my bad English and a lot of text...