Well I think Warlord should be a class!
The idea of a class that is about being....what? Badass? Inspiring? Is too limiting. I spoke some times of the equivalent of that class I have for my games: its called the Companion. Its not the Ulysses, the Jon Snow, the General Paton nor the Aragon of novels and movie that can better be replicated with other classes and some character building in game. Its the Samwise, the Sam Tarly, the Robin from said novel or movie. Its the type of character that help a group be more than the sum of their part, because its a world where, some times, the mundane wits and goodwill of the underdog CAN help even the most independent of heroes.
That's a kind of character you could do with the Warlord class, in 4e, usually with some of the build options that gave you the 'LazyLord,' It's a good example of, well, literally a sub-class a sub-set of the things the full class should be able to do. Of course, the class should also do the Bravura builds who /were/ badasses in their own right, too.
...ooh.. sub-classes...
The Warden, you usual quick witted, Int-based strategy/protector guy
The Emissary, the social specialist wanting to defuse situation instead of fighting
The Vagrant, the trope-y backpack guy who always have the necessary items on hands.
The Brave, that reckless friend we all have that have crazy plans that somehow always work.
Coming Soon: The Scholar, the wise friend who knows a bunch of stuff.
...let's compare notes!
Extant 'builds' from 4e:
Inspiring, the most literal 'leader' take and the concept most covered by feats &c in 5e, so far, all about CHA, best at restoring hps.
Tactical, the most military-leaning 'comander'ish take, MM's partial Fighter-sub-class design of the 'warlord' limited itself to this INT-based build.
Bravura, lead-by example combat bad-ass, a risk-taker who encourages the same in his allies.
Resourceful, planning an material preparation, down with Combat as War - and improv when the plan contacts the enemy.
Skirmisher, mobile scout/woodsman/guerilla, who can keep the party together & sticking to those tactics rather than go on ahead by himself.
Insightful/Watcher, observes & predicts enemies actions, understands their motivations
community ideas:
Icon ('LazyLord' - I like Garthanos's 'Princess Build,' but y'know, gender), it's not so much what the Icon does, as what they represent to the party. Can include the plucky side-kick who inspires by trying, even if he'll never be in the same league as his allies.
Marshal, commander of masses of faceless troops (could be a 'pet class' with a swarm-like template pet, or could be saved for a wargame spin-off, like it's namesake was in 3e)
Artillerist, an archer himself & an organizer of ranged-weapon & spell volleys
Teacher, advocate of a martial philosophy, not just practical tactics or techniques
Protector, the trope of the 'leader' who puts the lives of his allies first
Hector, inspired allies, but also goads and out-maneuvers enemies into making mistakes.
...
5e-style faux-MC sub-classes
Crusader - divinely-inspired to bring the faithful together for victory (cleric/palad)
Arcane Battlemaster - on the battlefield, leverages the most powerful of all resources: magic. (wiz)
Infernal Strategist - victory at any price, even your soul (warlock)
Thaneborn - traditional tribal leader who incites berserk fury in his allies (barbarian)
Ardent - telempath, projects inspiriation/courage/etc to his allies (Psion)