I often have that problem, though not to quite the degree you describe. I often stall on how to develop a basic idea into a meaningful adventure scenario. I can do it, but I get some really nasty Blank Page syndrome some days and need a kick to actually get down to work.

The most reliable tool I use for painlessly expanding an idea into a larger amount of playable material is…

Dr Rotwang's Adventure Funnel

Not to be confused with a “funnel adventure” which is entirely different, the Adventure Funnel is a GM preparation method for taking a seed idea and expanding it out into many ideas. The key practical effect it achieves is giving you obstacles and details to make your scenario more involved. The method it uses is a structured brainstorming exercise. I find structured brainstorming methods far more usable than just kinda trying to think of ideas without a structured method.

This is helpful because the more involved your scenario becomes, the more time it takes for the players and their PCs to step through its parts, and the more stuff happens during games. It might not seem like that should make much difference, but when you mix more stuff with a bunch of unpredictable people around the table making choices, you get a multiplicative effect, and the result is satisfying gameplay.

Making an adventure using the Adventure Funnel

Before you start, know that what you'll get out of this process is not a full adventure, but a pile of pieces that are designed to fully enable you to improvise a fun adventure out of some or all of them (to be determined during play), or enough pieces that you can shove them together and write out a pre-planned adventure outline without any major creative blocks to overcome, just polishing work and slotting in stats and names and such details.

The basic process is well-described in Dr Rotwang's original blog post, but can be summarised as three steps:

1. Write down your basic idea at the top of a page. You've already got that, so this is easy.

2. Write down 5 obstacles to achieving that goal. Not even a sentence for each, just a basic idea, short like your top-level original idea is.

   This shouldn't be that hard. If it is, you're overthinking it. Just throw down whatever you can think of. Get it done, don't try to get it perfect.

   Don't worry if you think an idea is bad — write it down anyway. You're writing down five of these: not every one of them has to be a winner or will necessarily get used.

Ha ha, you already have enough material for an adventure without even doing step 3! Good for you. You could be done now, if you want. But if you want, using that material will be easier at the table if you…

3. Note down some details. Do this numbered point-form again, just like step 2. Details are anything that fleshes out things you're interested in from previous steps. Don't force it, but instead go for the things you've already started thinking about or wondering about while you were getting down the basic idea and obstacles. Basically, you should already have some things you're itching to note down — this is where you do it, instead of cluttering the steps above.

   Details are things like names, motivations, scene-setting notes, people involved, objects, why or how exactly a step 2 thing is an obstacle, locations, etc. These are your notes for interesting details (literally). They need not be long either: a sentence or sentence fragment.

Now you're really done. You could run a whole session or three with the material you've got now. And introducing your material during play will lead to the players doing things that take up time and are interesting to narrate, resolve, and springboard off of, so it will actually turn into much more material than you actually prepared.

Optionally, add:

4. Assistance and rewards. Step 2 is all about stuff in the PCs' way. This step is about stuff that they might run into that can help them, and stuff that they might learn about that they can earn for dealing with the adventure or particular obstacles. This is where you make a note about the helpful oracle in the forest, the villagers who can give information, or the treasure that the orcs have been collecting.

A worked example of using the Adventure Funnel

Dr Rotwang's post not only describes the method, but gives a worked example I urge you to read.

But I also have a Dungeon World dungeon/adventure front I should be writing instead right now so, I'm just going to Adventure Funnel that right here and now. If it seems oddly non-fantasty in parts, that's because we're playing a Final Fantasy inspired game, so there's magitech and stuff in our world.

(My players, if you're so unlikely as to be visiting RPG.se, stop reading here if you don't want to be spoilered! Can't spoiler-block this much formatting. Also, you'd trip over anything that changes between here and actual play…)

A few notes on my thought process are in italic parentheses.

Goal: Retrieve a record of the carvings in the Tomb of the Forgotten Hero.

Obstacles:

1. The tomb is said to be guarded by the spirits of all the Great Beasts slain by the Hero. (I'm cheating a bit here: this is an obstacle the Bard's player gave me during our first session, when I asked.)
2. Goons from the Empire
3. The inner tomb is sealed with magics
4. Mana crystals have grown and are blocking passages (growing crystals are something I already knew were part of the world and this adventure, I just didn't know how yet)
5. EARTHQUAKE (Stalled on 5th idea, so I just wrote down the silliest/worst idea I could think of.)

(That took all of five minutes! Woo!)

Details:

(This is where you'll see me start making connections between things above, and showing which caught my imagination.)

1. The spirits are totally a thing, but have been only intangible/scary before. Now they will (merge and?) create a mana crystal body — BOSS FIGHT!
2. Empire goons are here looking for a mana crystal seed. They know this is a leyline nexus.
3. Mana crystal seed is what the ghosts use to build a body around! It's a power source! (I knew about the mana crystals before. I did not know about seed crystals used as power sources though! Now I have other ideas for the Empire Front's activities and motives…)
4. The tomb has old technology apparent in its walls and rooms, all non-functional now. Or maybe just dormant.
5. Oh, that robot with the pirates outside from the first session? Totally actually working for the Empire. Did it survive the explosion? (Because I'm prepping for Dungeon World, I'll leave the truth of that last point to find out during play. Also, if I have other Empire Goons show up, I don't need the robot as much if I want to make the Empire's presence felt. Now I got options!)
6. Robot can explosively “ejection seat” its head and fly it back to its masters as a last resort.
7. The carvings detail the history of the last time the mana crystals went nuts and started growing everywhere. They are in an old language that will need translating though. (Not bothering to detail why they're after the carvings, since we've already established that. It's an Undine ship captain who's paying them to bring the info.)
8. Earthquake will open rooms up into the tunnels below. Possibly dropping the PCs down there?
9. Tunnels below are caused by mana worms following the growth of the crystal out of the Glasslands. They eat them? (Eh, don't like this idea. But I writes it 'cause I thoughts it.)
10. Crystals are visibly, but slowly growing. They've blocked off some passages: little enough to squeeze between (but still growing!); completely blocking but can see through/past, and/or super-dense growth that will be hard to ever get through.
11. Boss monster is made of living crystal, like the Mage's hand. It erupts out of the ground like the crystals, just growing faster than they usually do.
12. Boss monster changes forms as different spirits defeated?
13. Boss monster has multiple heads, for each spirit?
14. Boss monster shows off crystal magic. Crystal magic SFX are light blue, like our Robogolem PC's power source.
15. Boss monster can control the crystals around the room!

(At this point I have tonnes of material, but I still feel the ideas flowing, so I'm not stopping. This is working well!)

16. Goons are obviously going to mess with the seed crystal that's grown inside the main tomb and thereby trigger the monster and get smacked by it. Heroes, do your thing! (Here I have a kind of set scene idea. Being Dungeon World, it might not work out that way, but now I have raw material in mind to exploit or remix if the DW rules give me the opportunity.)
17. Empire goons got to the island of the Tomb in some kind of flying hover thing. It's down on the beach on the opposite side of the island from the PCs' and pirates landing site. They broke in through the ceiling. Uh, why? Ah, to avoid the wards, that they knew about.
18. Wards are glowing red sigils on the main tomb slab door. They're glowing weakly though and some aren't glowing at all. (Bard might Spout Lore on this and I can tell how that's not normal?)
19. Mana crystals are translucent, iridescent (inside, not surface) blue and red. Obvs connection with other magic and magitech SFX colours, but details TBD in another adventure.
20. This Tomb is a leyline nexus. Remember that and the leylines that come out of it, if someone manages to do something cool that would let them perceive that magic flow. There are three small leylines that converge here — minimum to be a nexus, so it's a minor one.
21. Undine captain is working with/for an Empire mage who wants the carving info. Not disclosing s/he's Empire, though. A rival of whoever sent the Goons, so will be angry at the interference with the heroes, obscuring the Empire connection.

(This took quite a bit of time, but that's because I got carried away. Like Dr Rotwang's original, you could really stop at five or nine or whatever few details.)

Assistance and Rewards:

(I always have a hard time with treasure in Dungeon World because I don't improv this stuff well. I'm much more a fan of poring over D&D-style treasure chapters and finding cool stuff. So Funneling this up is actually super-important for me bettering my Dungeon World GMing.)

1. The mana crystal seed, obviously. It gives magic powers, per its colour, which can be improved over time and study and attunement. Inspiration: Secret of Mana weapons and mana seeds, except for magic powers. Just one person, can't be traded around. Write a Compendium Class for each shard. (Yes, obvs there will be others in the world!) This one is light sea green, like light in a kelp forest. Breathing underwater will def be a thing it gives. Maybe also calling schools of fish, swimming fast, etc. Or is that too small for a CC?

2. The carving's story. All about how there was a colossal crystal pillar/tree in an ancient civ's capital, that was tapped into for magic/magitech/power, which over the centuries shrank. They noticed too late that it was shrinking, and by then the collapse was inevitable. Magic disappeared for a time, and all the scattered (comparatively) little growths crystals all over the world did too.

   The mage will blurt a bunch of this out, absent-mindedly forgetting “underlings” are around.

3. Undine captain's payment for services rendered. 30 coins each and passage where they wish to go, as agreed, plus 20 more each (from the mage, ultimately) to keep the mage's translation to themselves.

4. A Goon might drop a nice sword. (If Goon lives/escapes, recurring rivalry as tries to reclaim it!) It has a tiny red crystal shard in its hilt. When lit on fire, the fire burns without fuel until extinguished somehow.

Pros and cons

The biggest pro of using the Adventure Funnel is that it's quick and can be used either to lay the groundwork for writing up a detailed adventure, or for improvising during a session.

I find it particularly good for generating material I could fall back on during improvised GMing, since I'm not obligated to use a particular idea if it doesn't end up fitting the way the session goes. I can pick and discard bits on the fly.

Another pro of using the Adventure Funnel is that it can be used as an input to other GM preparation methods, instead of using it directly as input to a game session.

• Have a blank map, and want to create a dungeon or overland adventure from it? Great, Adventure Funnel some ideas while looking at it, then go tackle filling out your map.
• Want to create a Five Room Dungeon, but need ideas? Funnel some up, then build your Five Room Method using your Funnel material for inspiration.
• You've played your First Session of Dungeon World and now you need to develop some Fronts? Great, take your First Session notes, and Adventure Funnel some of them. Now you've got some moderately complex ideas to just fill in the blanks of one or more Fronts.

The biggest con of using the Adventure Funnel is that you will often generate material you don't end up using. If you don't force yourself to stop with a few details and get carried away like I did above, that can take a lot of time. You can recycle that material for another adventure though, so I like to consider it a bonus, not wasted effort, and using the Adventure Funnel generates material fast enough that it's at least an efficient use of time. Your mileage may vary regarding how you feel about that though.

Now though, I'm off to think about a sandwich, and let this settle and age a bit, so I can distill it into a proper Adventure Front, or perhaps a Perilous Wilds Dungeon Record, later today or tomorrow.