Hi there, I’ve recently begun a huge west marches style game, and I’m gonna need a lot to keep my players happy. So I’m calling upon you wonderful people to give me some plot hooks. Level and setting are no object, just hurl anything you think might be interesting at me. 

Ok here's one's I've used in more open-world settings: 

• Dinosaurs from the mountains to the west have been spilling out into the sheep pastures outside town and attacking livestock. The villagers want to find out what's going on in the mountains that's driving these animals out of their habitat. (In my campaign: it turns out a dragon-worshipping cult fleeing a town on the other side of the mountains has made a base for themselves at the top. There, they focus on summoning special wyrmlings to our world through mortal sacrifice, to strengthen their patron's grip on the world).
• Trade seems to have dried up from the south, folks are very interested to know why this is, and why the baron's guards won't let anyone over the bridge to the south. (In my campaign: a powerful and scrupulous wizard has hired a goblin mercenary army to pillage the southern region of the barony. The wizard needs materials for his researches (silver, gemstones, labor, test subjects), taking them from the surrounding towns and letting the goblins keep everything else in payment, in addition to promising them more powerful weapons when his researches reach fruition).
• The mother of a sick child needs medicine from a shop the next town over, but a wyvern has taken up residence in the forest and is hunting along the path and making travel perilous. (In my campaign: once the characters get past the wyvern and to the village of Oakhurst, they discover the shopkeeper doesn't have the medicine they need, but knows of a magical tree whose fruit has powerful healing powers. This leads into The Sunken Citadel from Tales from the Yawning Portal. The shopkeeper will reward the players for news regarding her son and daughter, also adventurers, who went into the Citadel several days ago.)
• Several inhabitants of a city are convinced that the dark forest about a day's ride from the city is growing at an alarming rate, and in a matter of a year or two might consume the road and leave the city cut off. (In my campaign: the forest is full of Awakened trees that were given consciousness by an ancient civilization long gone. They experience time differently (being very old trees) and know little of the world outside (again, being trees). One faction among the trees sees the creation of the road as a hostile overture by mortalkind and have little conception of the idea that the ones who cleared the land to make the road are long dead, and the trees have declared war on the mortals and are on the march to expand their territory. A recent incursion of bandits trying to hide out in the forest have exasperated tensions between the different factions of trees, with the warlike trees holding the bandits prisoner, so they can be interrogated about the mortals' invasion plan).
• Rumor has it that the current Baron murdered his predecessor. (This hasn't played out in my campaign yet, but the party has heard the rumors).
• A wanted poster calls for the apprehension of another adventurer and all their belongings,  last seen X (this is from a different campaign, but this adventurer had laid their hands on the Sword of Kas, and shadowy parties want it without letting anyone else know what it is they're after, hence the 'and all of their belongings' part of the bounty).

Aside from these, I also have a travel table I use to determine what sort of things players might find in the wilderness by accident. This might include any of the other dungeons from Yawning Portal (regardless of level, i belive in letting players get in over their heads in an open world game, as long as they're aware but everything they find out in the wilderness will be tailored to them) as well as a few homebrew dungeons I've written, that way the players can actually explore and discover. 

Hope these help! 

• A group of other adventurers have been arrested and charged with a crime.   They claim they are innocent because they were under mind control spells.  You have been hired by their lawyer to investigate their claims.
• Rumors of orcs attacking a small town arrive.  When you investigate you bandits in the town have been raiding the orcs and they are retaliating.
• The kobolds have been driven from the mountains, opening up new areas for exploration.  You are hired to map out the area.

The main thing IMO is to throw more hooks than you need at the players, because they're not going to bite on everything. Develop the ones they like and make them important, and let the others wither. (If they're particularly cool, you might try a couple of aspects, but don't get attached.)

Some that come to mind. (If any of my players are reading, some of these may look familiar. The others are totally unrelated.):

• The emerging evil cult of unknown gods/great old ones/whatever
• The emerging cult/religion that's actually a scam
• Some kind of catastrophe far off that's creating refugees
• Same catastrophe seems to be spreading
• The party get asked to bodyguard an NPC for an unknown length of time
• Doppelgängers are discovered to be replacing NPCs
• Assassination attempt on an ally of the PCs.
• An NPC pays the party for protection on a trip, because travel is dangerous. It turns out people are trying to kill the NPC, who has "no idea" why.
• PCs come into possession of some secret as to how to safely traverse an otherwise-dangerous area
• The PCs, either the whole group or an individual, are being followed/hunted by others, and the PCs have no idea why

A backstory is important for a good roleplaying character, but I'd encourage or even require your players to create some people/places/events that are important to their characters. By including player-created content in the adventure, player investment skyrockets into the stratosphere, and said content can easily be used as hooks for adventure.

Some random examples:

• A character might have a bitter rival that they come across in a town they visit.
• A character enjoys a certain festival that only occurs once every four years, and the festival is canceled/crashed due to goblin invasions.
• A character's home town is leveled by a dragon,
• A character's parent(s) lose their home to a corrupt governor.

The best plot hooks are the ones the players create.

• A shifty looking character is seen in town trying to sell a perfectly lifelike statue of >insert a character's family member, loved one, or dog here<, looking terrified.
• There is something disgusting and abhorrent under the town, and it is hungry. It's also very polite and doesn't eat people, and asks the party if they can find a way to have better food end up in the sewer
• A mysterious stone figure offers them a magical item, and a dangerous deal:
  - The Stone King (homebrewery link, let me know if you use it and how it goes!)
• A strange vehicle is seen moving at random across the countryside:
  - The Ethereal Fisherman (homebrewery link, also please let me know how it goes!)
• A pair of fueding families have turned their petty squabble into a local festival (good for a downtime session).
  - The Festival of Petty Disputes (homebrewery, also let me know if you use it!)

(all of the links above are the results of my entering the Competition of the Finest Brews!)

I have 2 plot hooks that worked very well for my group:

1. Among the remains of an ambushed caravan, the players discover the bodies of some minor, local heroes (people who the PCs or locals might know by name or reputation).  Among their possessions is a note from an outcast noble begging them to go and reclaim his ancestral castle from the wild forests where it has stood abandoned for years.  It's said to be haunted, or even cursed, and the forest is known to be treacherous, but he's promised a generous reward if they can do it, and he's provided a map leading to the castle.  Since the heroes he arranged to accomplish the task are dead, the mission and reward are up for grabs.

2. A little backstory with this one: the players had run afoul of an organized religion, and in a confrontation with a sort of "inquisitor" cleric, the party was hit by a Banish spell.  To their surprise, they found themselves in the presence of the church's deity itself, who told them, "I have a holy quest for you, if you will accept it: I want you to destroy my church.  I taught my followers to love and accept one another, but now they're torturing people in my name, and my words no longer reach them."   The simplified version of this story arc was that a cult had infiltrated the church slowly over decades, and corrupted its teachings until they were now worshipping a fiend who granted the cult leaders power as they converted more unknowing followers.

I always thought the point of a West Marches style game was that were no plot hooks - parties just wander into the unknown and see what they find.  That said, I  ran a homebrew side-quest that was great for a low level party but could easily be graded up. It is set up as sort of a sequel to 'Spirited Away'. I gather there are Ghibli based adventures for 5e out there but I couldn't find them so I did this.  Sorry for the wall of text, but I'm old school pen and paper so no links.

The party crosses into a pocket of the Feywild through a tunnel.  In my game the party meet a disguised dragon whilst travelling with a caravan and she begs them to find her daughter who had disappeared whilst looking for her brother.  The hook is not necessary though and a party could easily stumble across this tunnel whilst exploring.

In my adventure, the party were looking for Haku's sister who had gone in search of him.  After not being able to find him (he had previously been rescued, of course) she had cursed the Bath House.  If you don't use the hook, they could learn this (and how to release her by finding her true name) by talking to her and the hag.  The frogs (from the movie) had all become Bullywugs and were situated in various places around the place.  The Bath House was run by a Green Hag who was desperate to return the place to working order - she wasn't really fussed how.  In my game, the party could either clear out the place of the Bullywugs through combat or uncover clues and figure out Haku's sister's real name (her name was also the name of a river - if they had a map, they could figure it out, there was a recurring nursery rhyme that would mention her name and a visual puzzle). 

There were mimics around the place and trapdoors that always sent the party members down to the boiler room (I was trying to teach them to be cautious - it took them 5 trips!) but also fits the theme of their being pipes leading up from there.  The boiler room had either smoke, steam or magma mephits (cursed susuwatari) or an Ettercap (cursed Kamaji) in there (regenerating randomly each time).  I also re-skinned a Shambling Mound as the polluted river spirit, but they did not have to reduce him to 0HP before they 'cleaned' him up. There were some other monsters/NPCs to fight/interact with as well reflecting the original movie (paper golem swarm, awakened vegetables, kitsune, were-pigs, etc. - homebrewed or taken from various sources).

There was a harpy (cursed Yubaba's bird) on the top floor protecting the hag's possessions. I considered putting some flying skulls in there as well (cursed Kashira) but ultimately didn't have to.  In the office was a huge chest of drawers that contained various treasures but the drawers had to be opened in a certain order (if the party were perceptive enough as they walked around the bath house they would have noticed that the rooms had numbers out of order - the chest of drawers is essentially a map of the Bath House).  If they open out of order there might be nothing or psychic damage. Opened in the right order they would receive trinkets, gold or the odd occasionally useful magic item.  If they find the name, the girl is released and the curse is lifted.  If they kill all the bullywugs they can ask the hag for the girl to be released or another reward.  The girl ,a dragon, if released could reward them or their mother could later on, if you used the hook.

It was a fun, whimsical take on a classic dungeon crawl and was fun for both those who hadn't seen the movie and those that had.  The hag and the girl/dragon gave a lot of opportunity for RPing and it could be run with minimal combat. I also had a good time DMing it and it took a lot of pressure off because it was so easily extended or shortened time-wise and could easily be levelled up on the fly if the guys found it too easy.

These are all fantastic! Thank you so much for the help, will deffo be using some of these in the campaign!