Add rules for mass combat, and bingo you got something that I think is pretty cool, and a lot more appropriate than a new class.
Yeah, that would have been fine, but as others have said, too many classes are too broadly designed.
This is an interesting problem, actually, it goes all the way back to 2E and Kits. What Kits had to work with, basically, were class features (including proficiencies and so on). They could add and remove from those, but rarely dared to go beyond that. This meant that some classes had a hell of lot more that a Kit could tweak than other classes. This is striking when you compare the Complete Bard to say, the Complete Fighter, or Complete Paladin. The Bard has a huge array of "knobs you can tweak" in the form of a ton of class features, so you end up with a very diverse, high-quality, and well-balanced bunch of Kits. The Complete Fighter has barely any, but the basic class is so solid, limited damage can be done, and the Kits are mechanically bereft, but conceptually okay. The Paladin, on the other hand, is absolute disaster, because it has surprisingly low number of knobs you can tweak and still keep it "basically a Paladin", and most of the knobs, by default, were set to pretty powerful settings so it's seems like it was difficult for them to do anything but weaken it.
With subclasses you face somewhat similar issues.
Some classes have a ton of their power wrapped up in their subclass, or subclass and some other choice-based-feature, but several other classes have the vast majority of their power in the basic class, with the subclass doing very little. And thus you can't get equal results by attempting to "Warlord-ify" classes. And really I can't think of any class in 5E that allows the subclass to have so much power that you can really go "full Warlord" with it either.