1) Class Specificity.
Because core 5e basically locked in on the 'generic classes' as hard as it could to be as flexible across settings as possible, you really need to make the classes you create, here, specific to a specific setting and/or structure. Well. Within Reason. You're still going to want the classes to be as broad as you can make them for narrative purposes, it's just not going to be as broad as "Fighter" or "Rogue".
2) Class Connection
By giving the classes a specific place in the world, with organizations, identities, and structures, you create elements that DMs and Designers can lift out, whole cloth or piecemeal, to drop into their own homebrew games. This is something most of us are 'guilty' of to various degrees, but who cares? If you design the connection to feel natural enough, or tropey enough, it'll fit in anywhere.
3) Conveying Class Identity
By introducing a bunch of new classes with somewhat more narrow identities than "Wizard" you need to develop a method of showing, not telling, what they're narratively 'about'. How their mechanics impact the world and the social differences in structure that go with that are super important. So having a setting where you can show their impact, their connections, and their identity is super useful.
Imma take a stab at this. Im gonna go for my pet project of social pillar focus. Im picturing something of political intrigue of courtly drama. Kings and knights and holypersons and jesters etc.. For funsies ill use Latin words for descriptions to give it some identity thats similar to known class concepts, but not entirely familiar. I'll call the setting "Terra Intrigarum" for land of intrigue.
To hit the three quoted targets im thinking of hybridizing classes in a way that Paizo did with PF1. Id also put a lot of effort into backgrounds and the skill system expanding it out form the basics of 5E.
Nobilis
The noble class of Terra Intrigarum (TI) a mixing of the monk and bard to represent the understanding of martial experience tempered with courtly customs.
This may seem like an odd pairing, but thats kind of why I like it. Nobles are often born in isolated circles and forced to learn skills of sophistry. Their martial skills are less those of heavily armored combatants and more improvised caught of guard environments. The bard aspects lead into the idea of social connection and protecting ones mind from probing inquiries. Nobilis are the heart of the realm, the decision makers, but also entrusted keepers of the peace. They are often targets of kidnappings and assassination attempts as their high station grants them such risks.
Eques
The knight class of TI mixing fighter and paladin to create capable defenders of honor and masters of the battlefield.
This is a lead into King Arthur romantic idea of knights as defenders of court and leaders of the battlefield. Unlike nobles, the knights of TI learn proficiency in heavy armor and weaponry as well as court customs of officers of the realm. Eques would be the open face of defense and honor (think perhaps samurai compared to ninjas). Their office is earned through dedication and well known across the court and beyond. Eques are expected to champion the nobles and their peoples with their lives if necessary.
Prices
The holy bound class of TI mixing cleric and druid to represent spiritual leaders of the Earth and beyond.
Prices are the spiritual leaders with insight into the Gods and their seemingly unknowable ways. Holy persons of TI are expected to be protect the spirit of the realm and its peoples through knowledge and skill granted through their devotion and will. During times of unrest and uncertainty, people of TI look to Prices for answers to the unknown, and are often held responsible for ill omens and occurrences but also celebrated during times of bounty and good fortune.
Spiones
The shadowy agents class in TI a mixing of rogue and bard to obfuscate and discover agendas and have an unseen hand in carrying them out.
Spiones often hold no rank or have public facing station (though that depends largely on the realm they serve). Often times disguised as distant noble cousins or lower officers of the realm, spiones work the edges of the court always looking for intrusive agendas or to carry out unsavory agendas of their masters. People of the realm often have no knowledge of who these unsung heroes (or devils) are amongst the court and people of the realm.
Veneficus
Practitioners of magical arts class in TI the mixing of wizard and warlock classes to represent arcane knowledge and mastery.
Venficus are masters of the arcane arts. They are responsible for protecting the realm from outside forces, as well as being responsible gatekeepers of arcane knowledge within the realm. When their services prove fruitful the people look upon them with wodner. In times of strife they are often looked upon with doubt or suspicion.
Externus
Those who dwell outside of civilized holdings class in TI a mixing of the ranger and barbarian classes to represent rural expertise and customs.
The externus are those more comfortable outside of courts and civilized places. They are respected as they live among the unknown forces of nature and serve as a first response of dangers to the realm. Despite respect for their lifestyle and services, not often trusted within civilized places and often the source of misunderstandings amongst the people and the greater outside world.
Thats the start of it anyways.