I'd be happy with France and Germany but whatever.
It'd get boring after a while. I feel as though one of the reasons NW gets a bit stale is because we only get 5 factions. I know it's more than most games get an all, but the addition of more than 5 (If that's even possible, I don't know if it is) would be cool.
My dream roster for this mod would be:
Entente:
Great Britain (And her Dominions)
France
Russia
Italy
Central Powers:
German Empire
Austria-Hungry
Ottoman Empire