Community wiki post. Everyone can add an answer containing a short idea for a campaign, be it an event, a characteristic of the world, an artifact or object, or an NPC character. Fellow masters can pick up the idea and develop their own story starting from this short description.
The great thing about CoC original game, being set in the 1920s, you can use historical events for the basis of some great adventures. Here are some "seeds" from the 20s that with a little bit of brain power can lead to some mythos activity:
1920: In December one of the most destructive earthquakes in the history of the world struck the Gansu province of China, killing between 180,000 to 200,000 people. Perhaps something the PCs are investigating triggered the eathquake (dholes? cthonians?) and they have to go to China to investigate, or maybe something the PLAYERS did while battling a mythos being caused the earthquake to begin with!!!
1921: Carroll Deering, a five-masted commercial schooner that was found run aground off Cape Hatteras, North Carolina, had it's crew missing and no explanation ever agreed upon as to why the ship was abandoned. Perhaps the players are hired by a family member to find out what happened, and stumble across cults of deep ones, degenerate Dagon worshipping pirates, or worse out in the Bermuda Triangle. Worse, perhaps a cargo on board (an idol, mummified hand, or alien artifact) carried or found by a crew member was the cause, and the investigators come into possession of it!
1922: In May of 1922, the british ship Egypt sank off the coast of France when it was rammed in heavy fog by the Seine. The ship carried 10 tons of silver bars and 5 tons of gold bars. Over a million dollars worth, most of it was recovered by divers in 1932, but perhaps there was a more sinister reason behind it's sinking....a sorceror controlling a tribe of deep ones caused the fog, ensared the captain's minds to make the boats crash, and now has had his deep one minions looting the cargo. The investigators may be hired by Lloyds of London (which had insured the ship) to investigate, and run afoul of the evil mage and his minions....
1923: Are you kidding? King Tut's tomb was opened! Anyone should be able to get a decent adventure or two out of that one!!!!
1924: Sir John Stewart of Scotland, a whiskey distiller and baronet, was found in Feb of that year shot to death in otherwise unoccupied Fingask Castle. He was deeply in debt and the verdict was suicide. Later that year one of his creditors also killed himself. I don't know about you, but I want to see what's going on in that "unoccupied" castle, or what might be underneath it. Maybe Sir John and/or his creditor were investing money in something unsavory, or accidentally ran afoul of the wrong people (?) who didn't appreciate their interest. Maybe the cultist villagers killed him off when he started finding out things he shouldn't have....?
1925: The Derbyshire Werewolf. Something black and of huge sized terrorized this part of England in 1925, killing dozens of sheep, and mysteriously disappearing. Thought by authorities to be at various times a large wolf, an escaped jackal (from a private menagerie), or a black hound, this is a perfect type of scenario for a "twist" to confuse the players. Instead of a werewolf, perhaps it's a byahkee, dimensional shambler, or a shoggoth that has been summoned by a crazy old coot. Or maybe the party just thinks it's something supernatural, when it's a nutty death cult practicing on sheep until they can finally start on people eventually. Or maybe it is indeed a werewolf, and the party has to find a way to put it down...
1926: Colonel Percy Fawcett, his son and a friend disappeared in the Brazilian jungle in 1925 searching for the Lost City of "Z". Lots of theories abound to this day what happened to them....in 1926 the PCs are hired by a publisher or the group that financed the expedition (named "The Glove") to find the truth about where Fawcett went, whether Z exists, or what Fawcett was seeking. Lost cities in the South American jungle just seem to ooze primodial danger and evil.....
1927: Hitler and other Nazi leaders are organizing rallies, promoting the Hitler Youth Movement, and perhaps, seeking out certain artifacts of evil? Obviously many of these are found by Hitler and used to bring him to power, but perhaps investigators managed to beat the Nazis to a few of them in Europe and prevent Hitler from gaining such power that the world course of history would have been forever altered....investigating groups must battle nazis and their agents throughout Greece, Italy Spain and elsewhere looking for items such as the original Golden Fleece, the Spear of Destiny, or Colada (lost sword of El Cid) all with mystical powers. Of course Adolph has a sorceror ally that can summon all sorts of nasties (byahkhee, deep ones, hunting horrors, etc) to attack investigators searching for these treasures.
1928: Danish training ship the Copenhagen goes missing after setting sail from Buenos Aires to Australia in December of this year. It is a huge ship with a crew of over 50 cadets. Anything from sea monsters, to deep ones, to aliens, to frenzied cultists might have attacked the ship and drove it to the bottom of the sea....or perhaps the crew is being held on a uncharted island, being used for unspeakable purposes, waiting to be discovered (or unfortunately joined) by some intrepid investigators hired by the Danish government or a private firm....
1929: The stock market crash. OBVIOUSLY caused by evil powers from beyond!!! Also, Peter Kurten, a truly demented serial killer (Known as The Vampire of Duseeldorf) committed horrific crime after crime all during the year until caught. What if Kurten was not alone, but part of some unspeakable cult worshipping Shub Niggurath or another dark god? Investigators called in by german authorities may soon realize Kurten is not the only killer about...and some of the other cult members are high up in German political society!
There's one adventure possibility for every year of the 20s, any good historical source could lead to even more!
If you're in the prohibition timeframe, this one may well be good for parties which have some form of law enforcement or investigator bent (PIs, cops, etc).
A speakeasy owner (whose primary non-wait staff may be comprised of zombies) is brewing his own special whiskey. The whiskey is laced with a chemical that, over time, causes the drinker to go mad, and eventually even turn feral. The reason the speakeasy owner is doing this is, of course, left up to the GM.
Note that I left the Old Ones out of this. Remember: Not every Cthulhu campaign should center around them -- if the players are always saving the entire planet (essentially), it will eventually get boring. :)
Small Seeds involving Mirrors:
If you want to be particularly obnoxious to your players, put them in the middle of nowhere... like, say, Anchorage, Alaska.
In the 1920's, Anchorage was mud roads, and a couple thousand people. A commercial port, a rail yard adjacent, It ran 1st to 15th avenues, and from N to A then Barrow to Gambell. Two church town... one at 8th & H, and the other on 5th and H. Airport between 9th and 11th, A-I... Nearest town was the City of Spenard, a mere 4 miles out L street, and home to Chilcoot Charlie's bar - still at the corner of Fireweed & Spenard. (And technically, it was in the city of Anchorage - Northern Lights Blvd, the next major street (2 blocks) south, is the dividing line)
Outside of that, next was Palmer, 60 miles away, with a native village, Eklutna, and it's Russian Orthodox Church and Russian Speaking Eklutna Tribe Athabascans, in the midway point.
In December 1920, the first police chief was appointed for the 4 year old town. He was murdered in February 1921, in the back stairwell of the Druggist's shop.
In the real world, it's still unsolved... but in your game, it could be the kick off of a wild-west-with-cars kind of campaign.
It's said many strange things are done in the land of the midnight sun... And you'd not be the first to set a Cthulu game in 1920's Alaska... but one of a very few.
Oh, and there have NEVER been Sherrifs in Alaska.... Only Police in the cities, Territorial Troopers (later State Troopers), and the G-Men: US Marshals, and the FBI. One Marshal wrote in a report, "Never met a N* I couldn't frame." (I have handled that document... Records of the US District Courts, at the National Archives regional branch in Anchorage.
Some links for your consideration in the matter:
Cthulhu adventure seeds:
Characters wake up in a mental institute. They don't remember how they got there, but they have some memories, and remember who they are. They know they have been investigating something... They start the investigation or try to escape, etc.
GM can involve some Old Ones in their memories or just make players (not characters) expect Old Ones have something to do.
After some action and adventure ;) characters get caught and put back in the hospital. A nurse starts the evening reading session. She reads a novel. That's when players realize that their characters are in fact nuts and their names and memories are from the book.
Or is it a conspiracy? ...
For a black-humor-style scenario, what about a modern-day shopping mall as the setting? It's not a new concept - it's appeared to great effect in a Discworld book, for one thing, and I got my original inspiration from a short story in which someone reached into a wardrobe full of clothing and touched something slimy and "other" - but it worked out to be a very entertaining setup for a one-shot game.
All it took was the basic "somebody's trying to open a Gate for [insert favorite Mythos creature here]," with the method involving cursed items strewn throughout a shopping mall, such that as each one was touched by a shopper they'd be drawn to the center [think Food Court, or for extra weirdness points, the photo-with-the-Easter Bunny or Santa Claus displays!]. The PCs are also shoppers - I cast them as a kind of B-movie/Buffy-the-Vampire-Slayer set of teenagers, but one could use any character types, from off-duty cops or military folk to absent-minded professors or bored suburbanites. The descriptions of increasingly creepy/disgusting Things crawling out of brightly-lit, shiny-new merchandise seemed to be very effective, and once the PCs figured out what was going on they came up with a truly brilliant (and very funny) ritual for sealing the Gate again. I'm hoping to run a follow-on scenario one of these days: "Return to the Mall on the Borderlands", perhaps!
Time doesn't exist. It's a human construct to enable people to process the world without going insane.
For us, this is but a philosophical discussion point, but in a game like Call of Cthulhu, it can be a real feature of the universe, and can have all sorts of dire consequences.