A possible plot is that the DDLC girls somehow end up in the Underground. Monika, being the self - aware one, might try to take control of the situation in a similar way she did in DDLC. But in the Undertale world, she has to deal with Sans, who can sense something is off about her. Meanwhile, the other DDLC girls are trying to find a way home while also befriending the friendly monsters like Papyrus. This could lead to a lot of comical and interesting interactions as the two very different universes collide.
The plot could involve a glitch that merges the two worlds. Undertale characters start to show up in the DDLC world and vice versa. Frisk could end up in the DDLC clubroom and be really confused. The DDLC characters could be scared or intrigued by Frisk's determination and ability to reset. And in the Undertale world, the DDLC girls' presence could disrupt the normal flow of things. For example, Monika's attempts to manipulate the situation could accidentally set off some of the more dangerous monsters in Undertale, leading to a whole new adventure to fix things.
One interesting plot could be that the main character from DDLC, say, the protagonist, gets transported into the Undertale universe. They could start in the Ruins and interact with Toriel. Since the DDLC character is used to a different kind of world, their reactions to the monsters and the unique magic in Undertale would be fun to explore. Maybe they could try to use their knowledge from DDLC to solve some of the problems in Undertale, like trying to understand the nature of EXP and LOVE in the context of their own experiences.
Well, perhaps Bowser somehow makes a deal with some of the more malevolent characters in the Undertale world, like Flowey. Then Mario has to not only deal with Bowser but also the new threats from the Undertale realm. He might have to learn about the concept of LOVE and EXP in the Undertale world to fully understand how to defeat his enemies. And along the way, he could meet Sans and Papyrus, and they could have some really funny exchanges due to their very different personalities.
One possible plot could be that Naruto, while on a mission, somehow gets transported to the world of Undertale. He teams up with Frisk to stop Flowey from taking over both their worlds. Naruto's ninja skills, like his shadow clones, could be used in creative ways in the battles against the monsters in Undertale.
I think Monika would be really interesting. In DDLC, she has that self - awareness and a certain level of power over her world. If she were in Undertale, her attempts to understand the very different rules of that world would be fascinating. She might try to use her powers to change things, but in Undertale, the concept of determination and the power of the player is very different. So seeing her interact with characters like Flowey, who also has some unique powers and knowledge, could lead to some really intense and thought - provoking scenes.
One idea could be that Spider - Man somehow gets transported to the Underground in Undertale. He might initially be confused but quickly adapts his powers to the new environment. For example, he could use his webs to swing through the caverns and his spider - sense to detect approaching monsters. Maybe he teams up with Sans and Papyrus to fight against a common enemy, like a new, powerful and evil entity that threatens both the Underground and New York City in some way.
Some concepts involve crossing DDLC with classic literature. For example, a crossover with 'Pride and Prejudice'. Monika could be like Elizabeth Bennet, intelligent and strong - willed. The interactions between the DDLC characters would then be filled with the same kind of social intrigue and romantic tension as in the original novel.
One idea could be Sans from Undertale ending up in the Marvel universe. He could use his shortcuts and bone attacks to fight alongside the Avengers against a common enemy like Thanos. His laid - back attitude would contrast with the more serious heroes in Marvel.
One idea could be that the Turtles end up in the Underground world of Undertale. They might have to navigate through the various regions, like Snowdin or Hotland, using their ninja skills. For example, they could use their stealth to avoid some of the more dangerous monsters. Another plot could involve the characters from Undertale being brought to the Turtles' New York. They would then have to deal with the Foot Clan and other threats in this new, strange environment.
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.
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.
Well, another plot could involve Sans and Papyrus coming to the RWBY world. Papyrus might be fascinated by the Grimm and try to befriend them, while Sans could use his powers to help the RWBY team in their battles against Salem. He could also share his knowledge of timelines with Ozpin.