The plot was originally planned to be about how the trolls got their colorful hair.
Originally, the trolls' traitor was Miss Guffin, Poppy's former nanny who raised her since she was a baby. This was changed because the filmmakers thought after a few glimpses that she was so funny that no viewer could recover after she sold out the trolls. Miss Guffin was split between Creek and Grandma Rosiepuff.
Although, a character resembling Miss Guffin appeared in the Trolls World Tour-related short Tiny Diamond Goes Back To School, who is the school's teacher.
The 20th anniversary party was originally going to be Poppy's coronation party, which was already shown in one of the movie's earlier trailers.
In relation to this, there's a deleted scene in which Satin and Chenille have finished preparing different dress choices for Poppy to wear at her coronation.
Chef originally had a Villain Song called "Bringing Back Happy" which was cut as the creators felt it didn't make sense for a Bergen who didn't exactly know what happiness was to sing about it, as well as due to questions about the lyrics being too dark for a movie aimed at children.
There's an animatic for a scene in which King Gristle Jr. and four Bergen Castle guards chase the Snack Pack on their way to escape Bergen Town. This scene strongly implies that all of the Bergens were originally supposed to be antagonistic, with Gristle being the Big Bad instead of Chef, and the plot for said concept simply involving around rescuing the Snack Pack and then escaping away.
Shortly after Dreamworks acquired the licence, there was a proposal to recast the nomes as trolls in the planned adaptation of the Nomes Trilogy, which at this point had been in Development Hell for years. This obviously never came close to happening, and prompted Terry Pratchett to shut the project down entirely.
Cooper would have had a longer preparation scene where he talked to his reflection on a mirror about his decision to leave.
He wasn't originally going to do a solo journey either, as early storyboards had him coming with Poppy, Branch and Biggie.
Poppy was meant to sing "Can You Feel It?" by The Jackson 5, when expressing her desire to unite with the other trolls.
Cloud Guy was initially the one with the "I'm about to have a baby" scene instead of Guy Diamond. He would have popped out three cloud babies (referred in the concepts as Windy, Stormy and Jr.), who are still in the movie as the family he starts with the cloud girl in the ending credits.
When the trolls come into Symphonyville, they would have been told by Pennywhistle that the loss of their string was her fault, explaining that she was distracted from watching the gate by her union mandating break and a vending machine that refused to accept the dollar bill she put in it; while she was gone, the rock trolls stormed the town and crushed all the classical trolls' opposition. At this point in development, the classical trolls were designed as closer to formally-attired Pop Trolls with coiffed hair instead of cherub-like.
Speaking of the Rock Trolls' ambush on Symphonyville, a brief excerpt of Metallica's "Master of Puppets" was meant to play during it, as shown in this storyboard video (at the 1:03 mark).
On their balloon ride, Poppy, Branch and Biggie would have come across Cloud Guy with his new girlfriend, the aforementioned cloud girl from the movie, who's named Janet here. They break up when he made comments about starting a family, with her hooking up with a thundercloud on the rebound. The thundercloud pops the Pop Trolls' balloon with a lightning bolt, and they slow their fall by blowing gum bubbles. Branch, having swallowed his, was saved by a butt bubble, which he pleads the others not to tell anyone about.
This would have segued into walking into Lonesome Flats, where they meet Hickory camping out. Poppy befriends him with an offering of cotton candy, and since he's going the same way they agree to go into town together.
There was a different meeting between Barb and the Bounty Hunters. She goes to a run down bar where trolls not of the main tribes listlessly play their music to no one, a fate Barb comments happens to anyone whose music isn't listened to anymore. Also, in place of The K-Pop Gang and Reggaeton Trolls were characters like Little Miss Polka, the Mariachi Sisters and the Barbershop Boys. After rallying everyone present against Pop to hunt down their string the Yodellers, introduced in shadow, boast that they'll be the ones to bring in Poppy and her string. They then demonstrate a yodelling performance that blows the Rock Trolls away, shatters all the glass (and bursts a tomato Thrash was about to eat) and collapses the whole building.
There's also concept art of various turns the story could have taken as it developed. Satin, Chenille and Cloud Guy were meant to accompany the group at one point, Vibe City would have been big enough to have a train, and the royal Funk Trolls were unrelated to Cooper, with a single king who was huge. At one point, Cooper almost became a black sheep as the tribe he was descended from would have had the aesthetic the Classical Trolls had and act completely different from him. Also, when Mr. Dinkles nearly drowned, instead of seeing a vision of Heaven, he would have seen one of Hell, with a bunch of other worms staring at him with glowing red eyes.
There were a number of unused character designs:
Prince D was originally going to look more like Cooper, with a golden cap and a goatee. This design was conceived back when he was set to be voiced by Chance the Rapper.
The K-Pop Gang had two main sets of unused designs. The first featured sets of both male and female members who were more detailed and had stereotypically Korean facial features. The second had designs pretty close to the final one, only with dark colors covered by several neon streaks.
Minor characters like Coral Blush, his parents, and Bliss Marina were first given more generic designs than the final ones they got. This appears to have come quite late into production, as Coral's original design can still be seen in Tiny Diamond Goes Back To School and merchandise for the movie.
A lighter example ocurred with King Trollex. Earlier renders of him featured different eye and mouth colors, but more notably a hoop-like necklace.
A heavier example with Queen Barb as she had a few different character designs with one for example, being based off of a bat-like Troll.
Some characters were originally given different names. Examples include "Spyke" for Riff, "Jax" for Chaz and "Mama Bootsy" for Queen Essence.
"Carol" was originally the name given to the minor Rock Troll child Amp, before the current Carol was conceived.
There was a Rock Troll named "Crunch" who was dropped from the movie, though his design was still used for generic Rock Trolls, and he has appeared in an unofficial "Look and Find" book produced as merchandise. He seems to have been a prototype version of Sid Fret. There were also Punk Rock Trolls meant to be in the film along with Biggie and Smidge look-alikes.
A few merchandise have evidenced that to infiltrate the Rock Trolls, the Snack Pack were originally going to wear disguises properly based on Hard Rock instead of Rock 'n' Roll.
DJ Suki was supposed to reappear but was dropped due to her actress being unavailable.
The Rock Trolls' originally had more sea-based creatures that they used, such as eels, crabs and stingrays.
According to storyboards, Fuzzbert was originally going to appear during the royal wedding of King Gristle Jr. and Bridget. However, he was eventually cut from the movie during productions. Likewise, Merchandise from Mattel seems to indicate Smidge was going to be in the movie as well but was cut.
They were originally going to be Bergens, and a source of conflict for them was them figuring out how to get more trolls for their essence, given that they were no longer seen as a resource.
Also, Chef was supposedly meant to be still alive, working as a Troll poacher and selling Trolls on the black market which is how Floyd got captured. There was meant to be a flashback showing how he got captured.
Velvet would've had a character trait of using big words to sound smarter than she actually is, usually mispronouncing and misusing them.
They were originally going to have a Villain Song of them explaining their evil plan, set to Lady Gaga's "Applause".
Another pitch video shows that Spruce/Bruce's name was originally going to be Selleck and he had a different hairstyle.
Vacay Island was originally meant to be inhabited by giant dog-like creatures.
BroZone was all meant to have different skin colours at one point in development.
An early draft saw Branch captured instead of Floyd and sees Poppy reuniting the band to save Branch.
The development of Trolls World Tour made some changes to the Party Crashers:
The Party Crashers were meant to be a troll-like species. Since World Tour was meant to make that revealed, they changed them to be distant relatives of the Bergens.
The group were set to be around for more episodes, but their uncanny resemblance to the Rock Trolls would've risked causing confusion with the movies, hence why they've been absent since "Party Crashed". They were still mentioned in the episode "Tour Guide of Duty", when Guy Diamond comments about them without saying their name, eventually hanging a lampshade with the following:
Guy Diamond: Whatever happened to that guy anyway?
There are some unused plot concepts from "Branchception" seen in the episode's storyboards:
Fluffleberry, which was first mentioned in "Fluffleberry Quest", was originally going to be mentioned again in the dream sequence, with a scene in which the dream versions of the Snack Pack tempted Branch to stay around giant Fluffleberry cakes.
While chasing Branch, the dream Snack Pack would have simply tried to catch him for a hug rather than play drums.
Branch would have attempted to wake himself up with cold water.
in which Satin and Chenille have finished preparing different dress choices for Poppy to wear at her coronation.