Good grief.
Look, as a niche, there is, at this moment, no actual non-caster support class in 5e D&D. None. Now, casters? We've got at least three (bard, cleric and druid) caster support classes. Two of those, druid and cleric, are effectively the same thing in the game world - one's a bit broader and the other is just a nature priest. The only thing that distinguishes a druid from a cleric is shape-shift, a purely mechanical difference. So, ten years into this conversation, are we STILL having to justify a niche that is not filled?
This is what makes me want to clean my ears with a fork when reading this conversations. We spend three quarters of the time just trying to justify the existence of the class, rather than actually discussing the class itself. Holy crap, there are weaker justifications for the existence of EXISTING PHB classes. And the goalposts keep moving. We have to define the class mechanically. Ok, ohh, wait, that's not good enough, now we have to justify the class in game fiction, completely ignoring the existing descriptions of the class from 4e. Once we actually do that, we're back to having to justify the mechanics, because critics conveniently develop amnesia and we go around the circle again.
Can we PLEASE actually discuss the class at hand? If you don't feel that there's enough to a warlord, fair enough. That's fine. Go start your own thread that we can safely ignore. For the love of little kittens, PLEASE STOP with the drive by edition war crap.
The niche martial support is weak is my main point. I am designing a warlord myself, I don't really agree with the concept (martial healer, a lot of the 4E powers etc).
I mean the class Toast is a niche that has not been designed but I don't think we really need a Toast class or a class that makes toast and can throw it as a weapon like the Xmen.
The main point is a few posters are throwing together words and claiming that is a strong niche. They're not most of the ideas are actually back grounds and with 12 classes in the PHB we do not need 12 1/3rd archetypes for each one.
I would focus on the 2 in the 4E PHB they are the strongest archetypes IMHO, get the base class sorted and then think about Bravura, perhaps a magical one. The archer one has been obsoleted, I don't think the lazy lord works in 5E.
The Bravura one might be the easiet to design. It doesn't need th Fighter/Barbarian/Paladin/Ranger levels of DPS but it should probably be higher than a Cleric for example. You could give it 2 attacks a round at level 5 (or a gambit a'la warlock) and a weapon style perhaps at level 1.
You need to have a short rest second wind ability you can stick on others but it needs to scale (like the fighters) and you are going to need more than 1 (perhaps an extra one at the same rate as cantrips scaling).
You also need another healing ability backed in perhaps at level 2 like a clerics domain perhaps a light cleric reversed to heal instead of inflict.
At level 3 you get a major subclass feature. This would be spells for a 1/3rd caster, and perhaps bard dice and BM dice. If you want more dice you can get them via gambits along with some gambits/exploits perhaps that allow attack granting+ rider. These are all short rest abilities and you could have 2-4 to pick from along with a gambit/exploit that grants perhaps 2 dice (bardic or BM dice).
So by level 4 perhaps you could have something like 6 BM dice and 2 exploits/gambits which allows 8 uses of attack granting. Its not quite at will but if the average fight is 3-4 rounds and you get 2 short rests you can use them a lot. If you used the short rest= 5 minute rule that functionally allows you to get an extra short rest or 2 which more or less turns it into at will. If you focused hard enough on it you might get something close enough to the lazy lord (someone else can update the 4E powers to 5E).
If you don't want to focus so much on attack granting pick a different subclass and you can still pick some exploits/gambits to enable that sort of thing you just won't have BM dice to play with trading them in for bard dice perhaps or spells+ lay on hands which combined with your baked in WL healing. Make some non magical bless effect type option a'la the NPC Knights that you can pick that is another thing you can do.
So there is your 4 best subclasses to build around IMHO.
I like the name Battle Captain for a subclass (perhaps the tactical one), the Marshall perhaps can be inspiring one, Bravura is better at combat and throw in a magical one and in effect you have non magical bard, Battle captain, the bravura and the 1/3rd caster one could be cleric or paladin spell lists the cleric one might get a domain feature (precedent in the Divine Soul), the Paladin one can cast spells from the Paladin list and gets lay on hands at level 1 (which removes things like disease).
If you get the 1st 2 subclasses right those next two would be easy designs you are in effect swapping out some class features for something else. That is easy most of it is in 5E somewhere. The hard part is writing a few exploits/gambits that perhaps can be class exclusive, kind of like Warlock invocations and some of the 4E powers can return and be beefed up if they are outright magical. Come and get it can be similar to suggestion and/or do other things as well as its 4E effect.
You could do an aura one as well updating the 3.5 Marshall as a subclass. Replace short rest type mechanics with auras and stuff like that so tats perhaps a 5th one. Level 1 abilities.
Grant 2nd wind /1 day
Tactical: intelligence to initiative
Inspiring: Cha to healing
Bravura: weapon combat style (enabling a decent archer one a'la 4th), don't include TWF as part of it a'la Paladin.
Divine WL Paladin: One lay on hands, cleric some domain feature.
Marshall: some sort of aura doing whatever.
Lvl 2 Cure 30' 2d8+level (short rest), 1st gambit/exploit
lvl 3 Major subclass ability
Lvl 4 ASI
Level 5 some generic ability+ subclass exclusive gambits/exploits.
Combined with a new exploit/gambit/power every time you level up.
