I'm not sure what you're trying to say about the ranking system. You already have to rank up before you can lead others. Nobody will follow you as a private.
As for the difficulty, I'm thinking that every side should have a set amount of soldiers on the field at any given time... let's say 100 just to get a number out. If each side has 100 soldiers out at a time, that means that if there's 10 capture points and each team holds five, each point has 20 soldiers on it, assuming absolutely nobody is attacking anything and you have the same amount of people guarding the frontlines as they do guarding the territories behind their lines.. Then let's say you have fought the enemy back to the last objective. You'd have your army fighting against 100 enemies on the cap since it's essentially their Alamo/last stand. That would be one way to increase difficulty as you win territories, at least.
This is definitely something worth to try how it plays out.
How about making it adjustable, with an options slider?
Well, actually, maybe it'd be incorporated with the difficulty suggestion.
You know, Easy means that there'd be 70 enemies at the end, Medium 85, Hard 100. That way, as the game progresses, it gets harder, but the difficulty "slope" is easier on Easy.