Perhaps it would involve the characters from Family Guy somehow getting trapped in the Underground of Undertale. Peter Griffin could try to befriend the monsters there, like Toriel, while Stewie might be intrigued by the unique magic system and try to use it for his own evil (or not so evil) plans. The different art styles and tones of the two shows would create an interesting contrast in the fanfic.
Toriel would also be an interesting choice. Her motherly nature could be put to the test in the rather unorthodox Family Guy world. She might try to take care of the Griffin family, especially the kids, in her own way. Her interactions with Lois could be really interesting, as they are both strong female characters but with very different personalities. And her fire magic could come in handy when dealing with some of the more dangerous situations in Quahog.
One idea could be a crossover with 'The Simpsons'. Imagine Peter Griffin and Homer Simpson getting into a wacky competition, like a donut - eating contest that somehow gets out of control. Another could be a crossover with 'South Park'. Maybe the kids from South Park visit Quahog and chaos ensues as the different humor styles clash. A third idea might be a crossover with a superhero universe. Picture Stewie teaming up with Spider - Man to save the day in a very 'Family Guy' - style adventure.
First, decide which other show or universe you want to cross over with. Then, think about how the characters from both will interact. For example, if it's a crossover with 'Teen Titans', consider how Robin's leadership style would contrast with Peter Griffin's laid - back attitude. After that, come up with a basic plot, like a villain from one universe threatening both. Start writing the introduction, introducing the two sets of characters and the situation that brings them together.
Well, it could be about the RWBY characters somehow ending up in Quahog. Ruby might befriend Stewie and they could go on some wacky adventures together. Maybe Weiss could have a culture - shock moment when she meets the Griffin family. And Blake could get into some interesting discussions with Brian about literature or something.
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.
To write this kind of fanfic, start with a strong hook. Maybe start with Steven waking up in the Underground or Frisk arriving on Earth near Beach City. Then, build the story gradually. Show how the characters react to the new environment and each other. Don't forget to add some action and conflict. It could be a battle between the Gems and some of the more aggressive monsters from Undertale. And make sure to stay true to the essence of both universes. Don't change the core personalities of the characters too much. This way, fans of both shows will be able to enjoy your fanfic.
The cultural clash. BNHA has a modern, superhero - like society with hero agencies and rankings, while Undertale has a more fantasy - based, underground world with its own set of rules and moralities. In a crossover fanfic, seeing how the characters from each world adapt to the other's culture is really interesting. It can lead to some really fun and thought - provoking situations, like how the BNHA heroes would deal with the monster - human relationships in Undertale.
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.
One great 'Frisk x Asriel' fanfic is 'A New Beginning'. It beautifully explores their relationship in a post - pacifist route setting. The story delves into how they help each other grow and understand the new world they are in. It has some really heartwarming moments where Frisk and Asriel support one another through various challenges.