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 creative storyline could be where the characters from Batim end up in the Underground of Undertale. Maybe Bendy and his crew are mistaken for strange monsters by the Undertale characters at first. They then have to navigate through the various areas of the Underground, facing challenges like the puzzles in the ruins or the battles in Snowdin, all while trying to find a way back to their own world. Another idea is that the Ink Machine from Batim somehow gets connected to the Determination power in Undertale, causing all sorts of wacky and unexpected interactions between the two universes.
Definitely. A good place to start looking is on fanfiction.net or Archive of Our Own. Look for ones with high ratings and good reviews. There are fanfics that explore how the different art styles and story themes of the two franchises can co - exist. Maybe a story where the protagonists of both worlds have to team up to defeat a common enemy that somehow combines the evils of both their universes.
The exploration of morality is a theme that can be found. In Undertale, morality is a huge part of the gameplay with choices between mercy and killing. In BATIM, there are also moral dilemmas in a sense, like whether to trust certain characters or not. Fanfics might combine these moral aspects, having characters from both face new moral choices in the crossover world. Also, the theme of discovery is there, as characters discover the new rules and beings of the other's universe.
Frisk might be curious about Bendy. They could approach Bendy with an open mind, trying to communicate. Bendy, on the other hand, might be cautious or even a bit scared at first. After all, Frisk is an unknown entity in his world. They could start by simply observing each other, with Frisk making small gestures to show they mean no harm.
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.
The plot might center around Frisk's sense of justice. In Undertale, Frisk often makes choices based on what they believe is right. In the BATIM world, they could see the wrongdoings and try to fix them. When they meet Bendy, Bendy could be a source of mystery. Frisk would try to understand Bendy's nature. It could also be that Frisk's ability to reset in Undertale has some sort of strange effect in the BATIM world, leading to all kinds of unexpected twists and turns in the story.
You can blend the two worlds by having characters from each world interact. For example, the ninjas of Naruto could meet the Berserkers. They might initially be at odds, but then find common ground in their fighting skills. The ninjas' use of chakra and jutsu could be compared and contrasted with the Berserkers' brute strength and rage - based powers.
Frisk and Dave Strider would be an interesting pairing. Frisk is all about kindness and determination in Undertale, while Dave is this cool, sarcastic guy from Homestuck. Their different personalities could lead to some really fun interactions, like Dave trying to be all cool while Frisk just goes about their business of being kind to everyone.
Identity is also a big theme. The undertale characters are put in new situations in the supernatural crossovers, and they start to question who they really are. For instance, if Chara is exposed to a world of angels and demons in a crossover, they might wonder if they are truly a 'bad' character or if they can change and find a new purpose in this new and complex supernatural setting.
I really liked 'BNHA: Undertale Edition'. In this fanfic, the author does an amazing job of transplanting the BNHA characters into the Undertale universe. They face off against the classic Undertale foes but with their own BNHA - style abilities. It's full of action - packed battles and some really heartwarming character interactions.