The color palette of both can be combined. Little Nightmares has a lot of dark, muted tones, while Undertale has more bright and distinct colors in some areas. In the crossover, you could have the dreary halls of the Maw tinted with a bit of the purple glow from the Underground. For the characters, Six's ragged clothes could be contrasted with Frisk's bright blue and purple striped shirt. You can also blend the spooky nature of Little Nightmares with the somewhat comical but also menacing aspects of Undertale, like making the Janitor have some of Sans' laid - back but dangerous attitude.
In terms of architecture, the Maw in Little Nightmares is a large, industrial - looking place full of gears and metal. The Underground in Undertale has areas like Snowdin which are more natural but also has buildings. In a crossover fanfiction, you could have parts of the Underground, like the Ruins, start to take on the mechanical and decayed look of the Maw. The doors could be rusted and have the same sort of ominous feel. And for the creatures, the faceless characters in Little Nightmares could be given some of the features of the Undertale monsters, like the glowing eyes or the unique body shapes.
A cool plot could be a character swap. Imagine Frisk from Undertale in the world of Little Nightmares. Frisk's determination would be put to the test in the nightmarish setting of Little Nightmares. They would have to face the Janitor and the Twin Chefs just like Six does. Meanwhile, Six could be in the Underground, dealing with Flowey and Toriel. The contrast in the two worlds' aesthetics and challenges would make for an exciting story.
Well, you could start with the colors. Godzilla is often associated with dark, earthy tones like gray and green. In My Little Pony, there are bright and pastel colors. You could have a scene where Godzilla is in the My Little Pony world and his presence creates a contrast, like a dark shadow against the colorful meadows. The ponies' cute and small structures could be compared to Godzilla's massive size, creating an interesting aesthetic blend.
One idea could be a crossover with Hollow Knight. The protagonist of Little Nightmares could find themselves in the Hallownest, facing the strange and often menacing bugs there. The dark and eerie atmosphere of both worlds would blend well.
First, decide which two universes you want to cross over. For example, if it's Little Nightmares and Alice: Madness Returns, study the lore and characters of both. Then, create a scenario that brings the characters together, like a strange portal opening up.
The character designs could blend in interesting ways. Take Alice Angel from Batim. She could be given a more Undertale - inspired dress. Instead of her normal white dress, it could be a dress with a pattern that resembles the Delta Rune. Her halo could also be a different color, like purple, which is a color often associated with magic in Undertale. On the other hand, Toriel from Undertale could be depicted in a more 'inky' form if she were in the Batim world. Her fur could have a wet - ink look, and her robes could be tattered like the clothes of the characters in Batim who have been through a lot in the studio.
One idea could be a crossover with 'Alice in Wonderland'. The protagonist of Little Nightmares could fall through a distorted mirror and end up in Wonderland. They would encounter the strange creatures there, but with a Little Nightmares' twist of horror and mystery. For example, the Cheshire Cat could be a more menacing figure, with a dark aura and a creepy smile that reveals sharp teeth.
First, decide on the two universes you want to cross over. For example, if it's Little Nightmares and 'Silent Hill'. Then, think about how the main character from Little Nightmares would end up in the Silent Hill world. Maybe they get lost in a strange fog and suddenly find themselves there. Next, introduce the elements of Silent Hill, like the rusty and desolate environment, and start building the conflict. Will the character be hunted by the creatures from both universes? That's how you can start.
For the main Jessies, the Jessie from 'Toy Story' is brave and loyal. The 'Jessie' show's Jessie is resourceful and kind - hearted. In a crossover, they could combine these traits. When facing a challenge like a bully in the penthouse or a mean toy in the Toy Story world, they'd be both brave in standing up to it and resourceful in finding solutions. Their kindness would also help them make friends with the characters from the other universe quickly.
One way to blend the themes is through the idea of sacrifice. In LOTR, many characters sacrifice for the greater good, like Boromir. In Narnia, Aslan sacrifices himself. A fanfiction could explore how characters from both worlds understand and are affected by sacrifice. Also, the theme of the power of nature is present in both. The ents in LOTR and the talking animals and plants in Narnia can be part of a story that shows how nature can be a force for good against the encroaching evil in both worlds.
In a crossover fanfiction, the settings can be blended by having a shared resource. Let's say there is a rare mineral. In the Firefly universe, it's needed to upgrade their engines for better speed and efficiency. In the castle world, it's used for some magical spells or to strengthen their defenses. So, both the Firefly crew and the castle's people are in a race to find this mineral. This brings the two settings together as they explore each other's territories in search of it.