Ranks would be a cool idea, but don't use BF2 as an example. As mentioned, the system was poorly designed and was abused to no end.
If ranks were added, I think there should only be four: The German and Russian equivalents of Private, Private First Class, Corporal, and Sargeant. Anything higher would be silly because RO is all based on small-unit actions. Make them only last as long as you're connected to one server.