Abandoned Mine: The third (and last) Carl Barks story featuring Flintheart Glomgold was about an abandoned gold mine being put for auction in Africa. Both Scrooge and Flintheart believed the mine to still have gold and Glomgold tried to prevent Scrooge from attending the auction. The story had an open ending as we never get to know who won the auction or if the mine had enough gold to be worth the trouble.
Always Identical Twins: Huey, Dewey and Louie naturally. It's especially evident in many of the comics storylines, as a lot of the time instead of their trademark red, blue and green the three of them wear identical black shirts.
Becoming the Mask: Has happened with Magica De Spell at least twice, each time under a relatively unknown author. A Gal for Gladstone (sometimes known as A Girl for Gladstone), by Carol & Pat McGreal, has her hex away Gladstone Gander's luck and then pretend to be an ordinary girl in order to get a shot at Scrooge's #1 Dime — she ends up sufficiently touched by Gladstone's sincere devotion to her that she ends up forfeiting the dime so she can save his life. Handled better, in some people's opinion, in Kari Korhonen's Date with a Munchkin, in which she kidnaps Daisy, takes on her shape, and pretends to be her, ending up chosing to stay at a Duckburg ball with Donald rather than go along with her original plan, willingly dispelling the illusion and leaving Donald because she can't bear to hurt Daisy by keeping him, and wondering to herself if what she got to feel during the facade actually makes up for the fact she still didn't get the dime.
Breakout Character: Scrooge McDuck started off as a supporting character / antagonist in a one-off Donald Duck story written by Carl Barks as a clear pastiche of Charles Dickens's A Christmas Carol. Fifty years later Donald Duck has shown trouble keeping his own title in publication, while Scrooge is the star of one of the two longest-running classic Walt Disney comic properties (along with the anthology Walt Disney's Comics and Stories).
Cannibal Tribe: These always show up in jungle or tropical island settings, especially in the older stories.
Celebrity Paradox: Some of Don's Hidden Mickeys refer to Mickey's real-life status as a fictional character, while the Ducks are "real" people. Take into account that Donald started off as Mickey's co-star in the cartoons, and you see how this fits.
Chased Off into the Sunset: Frequently pops up in the European Scrooge McDuck stories, usually with Donald or Scrooge chasing after various characters. When one of many plans to make more money fails, rather than accept responsibility, Scrooge blames Donald Duck. The story then ends with an angry Scrooge chasing after Donald, often carrying a big club or mace. Huey, Dewey, and Louie usually look on, sometimes with indifference but sometimes chortling with amusement, unless it was Donald's plan in the first place, in which case their uncle chases them twig in hand. And sometimes both happen at the same time, Donald being simultaneously the chaser and the chased. Other characters might appear depending on the story.
Comic Book Time: Most apparent with Scrooge's history in the Klondike, which was perfectly plausible when Carl Barks introduced it in the comics but would've meant Scrooge was over a hundred by the time of DuckTales. Some Italian stories play with it to imply that he is effectively immortal. For example a story with Scrooge celebrating the New Year of 2000, has a brief flashback with him celebrating the New Year of 1900.
Convicted by Public Opinion: A recurring theme. In Pool Sharks by Barks, Donald lets a couple of kids use his brand new swimming pool. This leads to dozens of kids getting wind of it, using and ruining the pool, which leads to their parents getting worked up about accidents happening to their kids, which leads to Donald closing the pool without ever having gotten to use it, which leads to everyone hating Donald. To be frank, the people of Duckburg are dicks.
Cosmic Horror Story: Yes, believe it or not, one of Donald's stories is indeed a classic Cosmic Horror Story. Donald tries out for a singing competition organized by a renowned musician, and gets successfully recruited by having his voice altered by an apparent twin of this musician. It's later revealed that the entire world is actually the dream of Ar-Finn, a primordial cephalophoid monster which slumbers in an ancient city at the bottom of the sea. The two twins are manifestations of the monster's conflicting subconscious desires to either continue sleeping or wake up (which Donald's voice will make it do). When the creature does exactly that, the rest of the world vanishes as it no longer creates the world-dream, and everything in its vicinity shapes itself into its image, resulting in Donald and his nephews growing tentacles and stick eyes. It's eventually put back to sleep, but the story ends on a rather dark note as Donald contemplates everybody's existence as mere parts of the creature's imagination.
Crouching Moron, Hidden Badass: Donald is the most prominent example of this, most notably when he changes into the Duck Avenger, though he has plenty of Bad Ass moments even when he's just himself.
Fethry is a less prominent version of the trope, as he usually succeeds by accident, but he has his moments of this as well. (and has a superhero identity, the Red Bat, as well, but one that's more of an Idiot Hero compared to the Duck Avenger's badass)
Even the Beagle Boys will, Depending on the Writer, sometimes display surprising competense and appear as a credible threat to Scrooge.
Depending on the Writer: Oh, so very much. For one thing, there's quite a few characters that only appear in stories by some authors whose existences are ignored by others, including cousin Fethry, Birgita McBridge, Donald's superhero alter ago, Paperinik, Scrooge's butler Battista, Scrooge's half-brother Rumpus McFowl, Scrooge's actual brother Gideon McDuck, John D. Rockerduck for most American authors, Flintheart Glomgold for most Italian ones, ectera...
One other thing that's wildly inconsistent between authors is the 'verse's relation to the Mickey Mouse Comic Universe. Some authors have them share a universe, but have the Mouse stories set in a different town called Mouseton, whereas others have both set in Duckburg same as the Duck stories. Some authors seem to set the stories in separate continuities. As noted under Celebrity Paradox, Don Rosa has an odd take on this: Mickey Mouse seems to exist within his stories... as a cartoon character.
Could be because as the Italian universe was set up during the Italian 50's, the era of neorealism, the Donald/Scrooge couple looks like the class struggle rather than Barks' Adventure Duo.
Detectives Follow Footprints: The comics get a lot of mileage out of this trope. For instance, they have a whole subtrope for characters exploiting the trope, knowing they are being followed, manipulating the footprints to mislead the pursuers.
Fiction 500: Scrooge, Flintheart Glomgold and John D. Rockerduck.
Fictional Country: There have been loads of these over the years as they're often disposable. Special mention must go to Barks for injecting real-world political satire into them, such as with Brutopia (a parody of the Soviet Union) or Unsteadystan.
"What did Scrooge and Goldie do in that shack that one night?", found in Don Rosa's own commentary. Cracked's 6 Insane Disney Comics You Won't Believe Are Real shows panels from a 1953 story "Back to the Klondike" where what at first looks like a fight between Goldie and Scrooge (given the sound effects and smoke emanating from the cabin) turns out to be something "not a hangin' offense in Langry, Texas, or anywhere else".
Half-Human Hybrid: your average person off the streets of Duckburg has a black button nose, may have unusually shaped ears and something of a snout but doesn't really resemble any known animal. Some stories by Barks have actual realistically drawn humans which makes things more confusing.
It can get even weirder when characters appear who are basically humans with a beak! (In fact, Gyro Gearloose comes very close to this.) Occasionally some colourist even has the gall to give such a character a human skin tone, instead of white feathers... Gyro has human feet.''...
Have a Gay Old Time: In the first The Three Caballeros comic Rosa did, he changed the lyrics of the eponymous song to remove the verse "the three gay caballeros". In the second one, the line is intact. You could almost swear there's a guy giving them a knowing gaze as they sing it that time...
Heads or Tails: In "Flip Decision", Donald is conned by a charlatan into believing in Flipism: the idea that all of life's choices can be made on the flip of a coin. Hilarity Ensues, of course, though the coin does show uncanny predictive power.
Insane Troll Logic: Italian stories like this. There's a story where Scrooge is continuously hit by lightning because he's "at the top of the world", a story where Scrooge moves in with a group of fishmen on the bottom of the sea to learn to handle deep sea pressure so he can cope with the pressure of being the world's richest, and a story where round-up unprocessed communal documents is the perfect retardant for a makeshift explosive because it's the slowest-moving anything in the universe. The stories just tend to handwave it off and ask you to pretend it makes sense.
Long Runner Tech Marches On: With the notable exception of Don Rosa, most Duck-writers let their stories take place in the present. Thus, while none of the characters has aged a day, the technology since the times of Carl Barks has marched on.
Lovecraft Lite: You could call Land beneath the Ground a Barksian version of H.P. Lovecraft, surprisingly enough - just read it. And while you're at it, check out Ancient Persia ...The Case of Charles Dexter Ward, anyone? None of these are gloomy enough to count as real Cosmic Horror Stories, of course.
Magical Native American: The Peeweegah, a tribe of long-nosed pygmy indians with the power to communicate with animals. First appeared in the Carl Barks story "Land of the Pygmy Indians", they then reappeared in the Don Rosa story "War of the Wendigo".
The Men in Black: Recent European-produced stories sometimes include the half-parodic T.N.T (Tamers of Nonhuman Threats), of which Donald and Fethry are freelance agents, dealing with supernatural or alien threats to humanity while trying to hide their existence to the common public. Unlike many examples of this trope, the T.N.T. are unmistakable good guys and do not wear shades.
In one T.N.T story, Donald got tired of the Boring, but Practical janitor-like uniforms and tried invoking a more traditonal MIB look by dressing himself and Fethry up in stylish black tuxedoes and Cool Shades. This phase lasted for exactly two pages, and was abandoned when Donald and Fethry discovered that the Cool Shades were too dark for them to see anything, and they walked straight out into a trafficked road. The results were Amusing Injuries and ruined tuxedoes.
Number One Dime: Trope Namer, with Scrooge treasuring the very first dime he ever made for an honest day's work. Because of Magica de Spell's avid pursuit of it to make an amulet that would grant her fortune, it often becomes exaggerated into being the actual source of Scrooge's wealth. Don Rosahated this interpretation of the #1 Dime.
Ironically for someone noted for favoring Continuity Porn from Carl Barks, he never did take note of the fact that Carl Barks actually did write a story in which Scrooge's fortune was aided by possession of a magical artifact; the 1950 story The Magic Hourglass.
He did, and mentioned it in his commentary of The Life and Times of Scrooge McDuck. As this was from the period when Barks was still only experimenting with Scrooge's character, and hadn't yet come to interpret him as the ultimate Self-Made Man, Rosa decided to quitely ignore this story in his personal continuity.
Putting the Band Back Together: In the second story featuring the Three Caballeros, Donald becomes especially depressed and Huey, Dewey and Louie decide to reunite the Three Caballeros in the hopes that it will cheer Donald up.
Remember the New Guy: A lot of characters have been introduced over the years, and several of them (especially the ones created by Carl Barks and Romano Scarpa) tend to be treated as if they've always been around, just not on-page.
A particularly noticable example is the Beagle Boys, who in their first story only make a silent cameo appearance on the very last panel... after Scrooge has spent the entire story worring about them.
The Scrooge: Guess who. His salaries to Donald and his closest workers are usually in pennies, and he'll do basically everything to not pay any service.
Secret Ingredient: In one comic, Donald insists on putting ketchup on all of Daisy's cooking, much to her annoyance, because it just doesn't taste as good as Grandma Duck's food. When Daisy checks with Grandma, it turns out the old lady's secret ingredient is...ketchup, which she puts in everything.
Self-Made Man: Scrooge. The point being that the "making" was more important to him than "getting made" in the first place.
Another one from Arpin : at one point, he refers to Scrooge as "Ze chipskate! Ze picsou!'. Now, "picsou" is not a french word for "cheapskate". It is, however, Scrooge's name in the french version of his stories (Balthazar Picsou).
Though the series in many ways is also great for aversions. As Scrooge is already the richest duck in the world, any treasures he finds will usually be a drop in the bucket. And so success or failure isn't as guaranteed as with other characters.
Tightrope Walking: A variation of this trope happens when Daisy Duck is on a building site and walks out onto a bouncy, springboard-like plank to retrieve a hammer left near one edge where it could fall on someone. She points out that (in this story) she's a ballet teacher and such perfect balance is nothing special for her.
Who Wants to Live Forever?: In the Tony Strobl and Carl Barks story, "King Scrooge the First", the reason the immortal King Khan Khan wants to find the lost treasure of Sagbad so badly is because it contains the antidote to the immortality potion he took when he raided the city centuries ago. He has grown tired of endlessly outliving everything and everyone dear to him, and after getting his hands on it, gladly eats it and wanders into the desert to join the dust that is all that is left of his civilisation.