When writing up the Marshal/Warlord for publication on DMs Guild, I had a short list of design goals:
1. It would be non-magical (martial as non-magical or mundane, not martial as a warrior). There are plenty of magical support class options.
2. It would have room in the subclasses for being an entourage or team class with mechanical support (magical options include ranger beastmaster, conjurer, necromancer, artificer/golem, paladin/steed, wizard/familiar, chain pact warlock). The mechanical support would be beyond what all classes can do with a mount, hireling, henchman, or pet.
3. It would have options to potentially recover the HP of others, aid the attacks of allies (or hinder the defense of enemies), hinder the attacks of enemies (aid the defense of allies), and help avoid or remove other harmful effects within reason. All with some advantage (however slight) to the general actions available to all classes and creatures.
In hindsight, I also added aspects of the 'Expert' NPC class from 3.x as each of the subclasses steer the class toward a specific skill set or proficiencies.
Entourage subclasses: chevalier and beast handler (commander and the others could be included if hireling or henchman were appropriate).
Experts: Healer - medicine, Diplomat - persuasion, Skald - performance, Chevalier - horsemanship and combat, Beast Handler - combat and animal handling, Commander - combat and leadership like the Battle Master and Purple Dragon Knight but on the Support/Encourager chassis.
I think there would be room in the subclasses for others like the Butler, the chemist, the Investigator, the Cook, and others. Kind of a Commoner/Expert class that could function as an adventurer by increasing the actions of the rest of the party while having a solid niche in interaction and roleplaying.