So I'm starting a campaign in a homebrew world, with the party at third level after having began another campaign that fizzled due to DM schedule change and I'm taking over with a new plot. The secret is I don't have much of an actual plot yet, and am hoping to get some ideas from how the party interacts with the world. I just need something to start with and I'm kind of drawing a blank. I have a module built for a mini-dungeon session to get them started, with the basic low level fetch quest premise after an NPC hires them to retrieve a valuable artifact that their scholarly organization just discovered the location of. They need to retrieve it rapidly because they believe some vaguely defined nefarious group may have also discovered it and could also be sending a retrieval team of their own.  Once they retrieve the macguffin they'll deliver it to the quest giver's headquarters in a steampunkish city built into the side of an active volcano and get some kind of follow up adventure hooks. 

But my writer's block is making me stall at the first step. What is the macguffin and why is it important? This is as much for myself to have something to work with building material for the next session. Presumably some sort of magical item, possibly needed for some sort of ritual, politically valuable, or...yeah, that's why I'm asking for you fine, creative, and possibly crazy folks here on yon interwebs of nerdery to help me spitball some ideas for basic plot hooks. I have a number of civilizations and locales in mind for the setting but have intentionally left any political relationships vague for flexibility once I actually get a plot to work with, so no real creative restrictions exist on that front. Feuding noble houses or mercantile guilds wanting some kind of edge over each other? A shady cult? An ambitious ruler looking to become an expansionist emperor? Fantasy mafia? Something that could somehow prevent a potential war which some faction seeks to profit from? A component for a mystical device that produces an unlimited supple of cheese that tastes like ham? I'm at the point that I almost want to contrive a reason for the ham cheese thing to be viable, so I could really use some help here.

You've got a steampunk city in a volcano as the quest giver, so it has to be something which would appeal to them.

The quest givers are the artificers guild. The macguffin is an enchanted workbench which creates anything that the user is focused upon making. They need to concentrate for an hour, after which the item they are trying to create, which cannot be magical (but can be later enchanted) finishes forming on the workbench. The artificers want this so that they can create new machines faster for the improvement of the city. 

Later on, the nefarious powers can attempt to get the workbench to create intricate and dangerous weaponry. They get hold of it and start amassing the weapons they need to take over the city - this could be for cultists to wield, or it could be more similar to the golden army from Hellboy 2. They combine the workbench with a magical item which has the ability to imbue the items it creates with magic, upgrading it to creating magical items as well.

Then something happens. The workbench somehow gains sentience and goes rogue. Being sentient, it can now create its own items and imbue them with magic. It effectively becomes the start of a macro-version of the grey soup scenario of the end of the world, but with self-replicating magic-item-making constructs instead of nanobots. These have built the workbenches into golems so they can move around.

Now the party had to fight a slowly growing army of magical constructs who can make anything, magical or otherwise, in an hour. They have to find and destroy the original workbench so that everything that it created will fade away.

You can spread these out with other things in between so they level up accordingly. Give the artificers time to improve the city, give the nefarious powers time to build their army, and give the sentient workbenches time to grow in power. For added twist, work out some things which will fade away as well, which were made by the workbench in the past. Perhaps a magma-dam in the city, so by destroying the workbench the city will also be destroyed. Establish early that the dam was made by the creator of the workbench (have them be a famous artificer from history) and see if the players pick up on the connection before it's too late.

The quest giving "agent" is based in the volcano city, but their order has other chapters elsewhere. Some good ideas though, still trying to brainstorm so more suggestions would be appreciated.

One thing that comes to mind is that you should probably think about whether your players are likely to take the MacGuffin for themselves and ignore the quest they were given.  If the object is powerful enough, or if it can be used easily, they might want to keep it!  Obviously this scenario could be just as fun as the original, but if it catches you off-guard, it could be difficult to adapt to in two minutes.

As for the MacGuffin itself, a book in an obscure language or code might be interesting.  Or a pair of sentient shoes that they have to chase down and capture to be studied.  Or maybe it’s a sample of a magical fungus thought to be extinct, which gives the user the ability to walk in dreams, and now the organization wants to study and cultivate it.  Or even some substance recently discovered to make amazing fuel.

 I’m not really familiar with the steampunk genre, so I don’t know how helpful my suggestions might be, given the theme.

What if said “scholarly organization" is the nefarious group and hires the party to retrieve the last component needed for a plot to gain control over the volcano and therefore the city. After they’re successful are they rewarded then have to thwart an assassination attempt on themselves? When the city is held for ransom do they realize their part in the plot and volunteer to help to atone for it, or are they arrested by the city guard for there involvement? Are they promised freedom if they help defeat this group, or do they have to escape to prove they’re innocence? Perhaps, unbeknownst to them, the item was the heart of a clockwork dragon, after they foil the plot (and make powerful enemies) do they go in search of the dragon (or more pieces) and try to reanimate it, or is the dragon evil and do they have to find a way to destroy the heart before someone else tries to awaken it?

Okay, now I'm getting some ideas...I might be able to work with some of that.

Mine is a doomsday device. Actually, originally it was a prototype of a doomsday device. If the bad guys got their hands on it, as well as the secret books of the inventor, and/or the inventor himself, they could learn how to build the real thing.

My players have not shown a lot of curiosity about the device, and the bad guys have obtained the books and the inventor, so the device will be going off very soon, and the players will have to find powerful ancient magic to stop it.

The device will create a gradually expanding void that at first swallows a city, but in a few months will be expected to swallow a whole continent.

I think it should be a magic item like a necklace of fireballs that would seriously tempt the PCs. Fireballs would also work with volcanos for plot reasons. Maybe they wouldn’t know it’sa necklace of fireballs until they bring it to the contact. Then they could get into heist mode. 
Or maybe a powerful device the players don’t know everything about- and then we need to consider whether or not volcano dude is the right person to have it. The PCs might wonder if he’s planning on using it for good or evil…

You could look at some famous examples of MacGuffins in other media and get some ideas from how they are used there. 

One very famous example is the maltese falcon, which was a MacGuffin that drove the plot of the film of the same name. Basically the falcon was a lost artifact of such immense value that it got the attention of various less-than-scrupulous figures who wanted to find it. Over the course of the film people are literally murdered over this falcon, and ironically

 Maybe you could do something similar. Obviously you'd have to adjust it to work with a high fantasy setting instead of a 1940's hardboiled detective story but what about some sort of legendary elusive artifact? Not even magical, just one that has become super valuable and is being sought after by various parties. You could have the party caught up in the efforts of some unscrupulous characters willing to do anything to obtain it. Maybe even caught in the crossfire between multiple competing parties. If you're feeling really ambitious you could even throw in a similar twist that makes the efforts of your antagonists seem petty and futile.

Whatever the macguffin winds up being, there's nothing better than players taking an item only to realize it's sentient and starts talking to them. Is it the wizened soul of an old wizard? A devil? Maybe the embodiment of a lawful god? 

Maybe too over the top for an early level fetch quest, or *maybe* just what this party needs to kick it into high gear.

YEEEAAAAHHHH *guitar solo*