This would make the game so much better. Don't just add the British, Americans (who didn't even do THAT much compared to most others) and Germans, add some more unique countries. Judging by the trailer, I can guess the Ottoman empire will most likely be there, which is a very good start. You can also include Austria-Hungary, Bulgaria, France, Belgium, etc. Please, don't make it the clichéd shooter where it's mostly good guys (Americans, sometimes Brits) vs bad guys (Germans, typically). Add more unique factions, and make them actively take part in the conflict, as they actually did in real life.
Hell, if you want to make it more interesting, you can straight up make it Allies vs Central Powers, instead of the usual Country A vs Country B. Many Allied countries' soldiers actually fought together in numerous battles, same goes for the Central Powers, so the realism will still be there.
EDIT: Corrected some stuff.