I could see this being useful and relevant as an addition to the regiments/crews feature, where instead of a big long list of everyone and one leader, the leader gets to designate ranks to the soldiers in his regiment and those ranks are shown on the outfits in-game, or at the very least maybe an icon above or below the player's name when you look at them in-game.
It could also go the way of NW and North&South where there was a 'serjeant' unit who sometimes had slightly different equipment as well as a different look, but I think it would be a lot more easier for the devs and for players if the regiments/crews feature handled the rank distinctions, they wouldn't have to make whole new classes of units, just adding some slight changes to the uniforms to reflect rank. It could also serve to promote joining regiments, or at the very least starting small regiments with friends to get the ranks on their sleeves, which could then merge into bigger regiments if they wanted the full experience.
Actually, on the note of promoting joining big regiments as opposed to making small friend regiments, there could be player count requirements in order to give out in-game ranks: a regiment of 3 friends cannot have anyone higher than the rank of maybe corporal, and maybe you would need a total membership of 15 to be a captain, 25 to be major, etc. I mean people will still call themselves Colonel of the tiny 3 man line, but they won't have a spiffy in-game colonel's outfit (which should be really darn spiffy, yeah? Like engraved pistol and sword, huge peacock hat feathers level spiffy).