Have you seen the number of homemade versions warlord?
A warlord in 5e could be like mixing BG3 and Age of Empires, or Age of Myths.
Other option could be start from zero with a no-D&D videogame, and later with the experience to be adapted to D&D. It could be something style "Reign under Fire II" or the saga "Heroes of Might and Magic". We don't need an AAA game but something to be played in tablet.
* Have you thought about how would be a fight against a warlord boss nPC with all his buffed minions?
The class features shouldn't be only right-balance, but the gameplay has to be fun, easy to be understood and simple for faster combats.
And a warlord with squads working as monster allies... a lot of troops for dungeons with small space or too cannon fodder against certain boss monsters that have been designed to fight against PCs with a lot of hit points.
A warlord class, and all the subclasses, have to be useful in the mass battlefield, but also in dungeon-crawler adventures.
It has to be designed for low-level-magic settings like "Game of Thrones" but also for players who wanted a harem of monster girls.
What type of characters from fiction could be D&D warlords?
The Warlord - TV Tropes
* A psionic mystic is not only a class, but the list of psionic powers.