If you want to make your Dungeons and Dragons world feel immersive, variety is key. If everyone is either a lord, a knight, or a peasant then things can feel a bit one-dimensional. It doesn't need to feel like that, though!
There are hundreds of different kinds of profession that you can introduce into your world - each of which can be a starting point for creating a unique NPC or an interesting sidequest. Here's a few ideas:
1. Messenger
In your fantasy world (just as in the medieval world), there would have been a need for messengers. Sending stones may be rare or expensive (depending on how common you want to make magical items in your world) so it's likely that there would still be a need for some messages to be carried by hand.
Plot idea:
Your party stumble upon a horse without a rider in the middle of nowhere. There is a messenger's satchel in the saddlebags, and officially sealed documents. One is for the king, and tells how strange folk have invaded the kingdom's southern border.
2. Cooper
Everyone needs barrels. Where else are you gonna put all the ale for the local tavern? Making them isn't easy though - they can warp or leak or split. That's why you need specially trained people to construct them. Sound boring? It doesn't have to be.
Plot idea:
This cooper makes barrels for the kings navy. Secretly, they are using a special varnish on the inside of the barrels that will slowly degrade and poison the troops. It turns out the cooper is part of an underground resistance network, working to overthrow the monarchy.
3. Witch hunter
Depending on the fantasy world you're creating, you may or may not want to go down this route. If D&D is an escape for you and your friends, maybe you don't want to recreate a world filled with persecution. Fair enough.
If you're ok with that, though, then it's up to you what you do with this and how far you take it. If in your world witches are commonplace and accepted, though, you could easily replace them with any number of different races or classes.
Plot idea:
The local witch hunter is running to become the town mayor - and is using his position to accuse and then burn at the stake any political opponents under accusations of witchcraft. The election is next week, and everyone in the town is too frightened to speak out.
4. Butcher
Well, where else are you gonna get your griffin steaks from? Most settlements could be expected to have a butcher - and they'd likely be well connected with other members of the town/ village. This would make them a good source of information when going out on sidequests.
Plot idea:
This part of the kingdom suffered a famine over winter due to a failed harvest. Vagrants were attracted to the town as there was news that there was still a plentiful meat supply here. Little did they know that this butcher was morally unscrupulous, and that the vagrants themselves were the source of the town's meat supply. Are the rest of the town unaware of what's going on, or just turning a blind eye?
5. Blacksmith
Again, a very common profession. All but the smallest settlements will likely have at least one blacksmith - whether for fixing farming equipment, making nails for construction, or forging weapons for the town guard. If you want some bespoke weapon upgrades, they're the person to talk to!
Plot idea:
This blacksmith has discovered a new way of smelting steel, to make both lighter and stronger weapons. He is searching for a financial backer who will enable him to further develop these weapons, and expand production to start selling them. Maybe one of your party is interested in going into business with him?
6. Navigator
A less common career path here, but still a very important one. Whether by land or sea, navigators would be hugely important in times where most people stay close to home throughout their lives. If your party are trying to get somewhere hard to find, a navigator may be invaluable!
Plot idea:
This particular navigator is searching for crew and a financial backer to charter a ship to a strange island a few miles off the coast. When sailing past it on a recent voyage, he noticed his compass and other tools all began to malfunction. He wants to know what is on that island that caused them to act so strangely.
7. Lumberjack
While many people would go and chop their own firewood, most larger villages and towns would be able to support a specialised lumberjack. They could make a fun NPC - as delving deeper into the woods than most others in the area, they might discover any number of mysterious things.
Plot idea:
A local lumberjack has refused to go back to work. The last time he was in the forest he saw giant tracks and several dismembered owlbears. Whatever is out there is big. And hungry.
8. Bee keeper
Even if you're not a fan of honey, surely everyone loves mead? Bee-keeping would be a very common profession, then. In the medieval world, this task was often linked to monastic orders - so perhaps it would be the same here. In a world filled with magic, however, what effects might magical plants have upon that delicious honey...?
Plot idea:
The local bee-keeper can't work out why suddenly his honey is having unpredictable magical effects. It turns out that somebody close by has been secretly growing and selling magical plants, and the bees have been collecting their pollen.
9. Brewer
Again, in small villages, brewing might be done in the home by each individual family. In larger population centres though, specialist brewers would be needed to supply the various taverns. Given that adventuring parties love to meet in those establishments, the local brewer might be privy to a lot of important gossip.
Plot idea:
A local brewer has been experimenting making new kinds of ale. Unfortunately he has no idea of their alcohol content. Anyone who consumes one must roll a d100 - this gives the alcohol percentage in the brew.
10. Dragon trainer
Ok, so this will be a pretty niche job, and not the kind of NPC that you'd stumble into every day of the week. Some people might also say that by making dragons trainable, you are undermining their mystique and the fear-factor surrounding them. Fair enough. You do you. But it's your world, so you make the rules. Also, humans have trained all kinds of intelligent animals - so why not dragons too? Might not be easy, but that's what would make this NPC such a badass!
Plot idea:
This NPC had been training dragons to get them to fight in the King's army. One, though, has escaped. The trainer must find and return this dragon, or face execution. They can't do it alone, though.