You could have the characters from Undertale drawn in a more FNAF - like style. For example, make Sans' eyes glow like the animatronics' in FNAF. Or give Frisk a more robotic look while still maintaining their Undertale charm. The backgrounds in the fanfiction could also be a blend. The colorful and somewhat cartoony Underground of Undertale could be made darker and more spooky like the FNAF settings, with broken animatronic parts scattered around.
In a crossover fanfiction, when it comes to combining art styles, think about the color palettes. The FNAF world has a lot of dark, muted colors with bright, creepy eyes or lights. You can incorporate this into the Undertale characters. For instance, Papyrus could have his bones be a bit more dull - colored like the metal of the animatronics, but his scarf could be a bright red that stands out like the FNAF signs. Also, the way the characters move can be a blend. The jerky, mechanical movements of the FNAF animatronics could be added to the more fluid movements of the Undertale characters in a spooky way.
The art styles could be combined by having the characters keep their distinct looks but interact with environments that blend both styles. For example, Cuphead and Mugman could be running through a hall in the Batim studio that has the dark, inky look of Batim but with Cuphead - style geometric patterns on the walls.
Well, you could start by taking the more angular and sleek design of Birdy the Mighty Decode characters and blend them with the softer, more fluid lines of the other crossover franchise's art style. For example, if it's a crossover with a more cutesy anime, you might soften the edges of Birdy's mecha slightly while still maintaining its overall shape.
One idea could be that the characters from Undertale find themselves in the FNAF pizzeria. Sans could use his shortcuts to avoid the animatronics' attacks. Another idea is that the FNAF animatronics end up in the Underground. Freddy might befriend Toriel, and she could try to 'fix' them. Maybe Papyrus would be terrified of the animatronics at first but then try to befriend them with his overly friendly nature.
One idea could be that the characters from Undertale end up in the FNAF world. For example, Frisk could be trying to find a way to escape the haunted animatronics while also befriending some of them in a similar way they befriend monsters in Undertale. Sans could use his shortcuts to outwit the animatronics' movements. Another idea is to have the FNAF animatronics transported to the Underground. They might be seen as strange new 'monsters' and the Underground residents would have to figure out how to deal with them.
One way is to use the color palettes from both games. For example, Undertale has a more muted color scheme in some areas, while Deltarune has brighter and more distinct colors in places. You could have scenes where the Deltarune characters bring their bright colors into the Undertale world, creating a contrast. Like when Kris enters the Ruins in Undertale, their blue and purple color scheme stands out against the grays and greens of the Ruins.
Well, it could be that there is a glitch in the multiverse that causes a fusion of the two worlds. So we have characters like Frisk interacting with Freddy Fazbear. Frisk might try to show the animatronics a different way of living, more like the pacifist way in Undertale. And in the process, they all have to deal with the threats that exist in both worlds, like the purple guy from FNAF and the more malevolent monsters from Undertale.
A possible plot could involve the souls in Undertale having some connection to the spirits that are rumored to be in the FNAF animatronics. The main character from Undertale, perhaps Frisk, could enter the FNAF world and try to free the trapped souls. In return, the animatronics could help Frisk in the Undertale world against some powerful foes.
The art styles can be combined by taking the detailed, gritty textures from Wolfenstein and applying them to RWBY's more anime - inspired character designs. For example, the characters' clothing could have the worn - out look of Wolfenstein while still maintaining RWBY's unique color schemes.
Frisk could be a very interesting character to focus on. Frisk has the power of determination, which could be used in unique ways in the FNAF setting. They could befriend some of the animatronics, using their kindness and determination. Also, since Frisk is the player - controlled character in Undertale, it would be interesting to see how they adapt to the more dangerous and unpredictable world of FNAF.
I think Freddy Fazbear and Toriel would make an interesting pairing. Freddy is this big, imposing animatronic, while Toriel is a motherly figure. Their interactions could be really cool, like Toriel trying to understand what Freddy is and Freddy being a bit confused by Toriel's kindness.