Perhaps 'Touhou in the Underground: A Crossover Fanfic' would work well. It gives a sense of the Touhou characters being in the Undertale setting.
A more detailed title could be 'The Convergence of Touhou and Undertale: A Fanfiction Exploration'. This title not only indicates that it's a crossover but also implies that the fanfiction will explore the combination of the two universes. There are many elements in both Touhou and Undertale that could be interesting to combine. For example, the unique characters in Touhou with their distinct abilities and the monster - filled world of Undertale. The fanfiction could explore how the characters from Touhou interact with the characters and environment of Undertale. Maybe some of the Touhou characters could use their powers to solve problems in the Underground, or they could have cultural exchanges with the monsters there. This title sets the stage for all these possible storylines.
A plot could be that some of the Touhou characters accidentally end up in the Undertale world. They then have to find a way back home while also getting involved in the conflicts between the monsters and humans in Undertale.
Another plot could involve a crossover of the two worlds' educational systems. The students from Ansatsu Kyoushitsu might attend classes in the Touhou world, learning magic instead of assassination. And vice versa, some Touhou characters could come to the school to learn about modern - day combat skills and strategies.
One possible title could be 'My Hero Academia in the World of Attack on Titan'.
In 'touhou crossover fanfiction', adaptation is a big theme. The Touhou characters need to adapt to the new environments of the crossover universes. Whether it's a high - tech sci - fi world or a medieval fantasy land, they have to learn new skills or use their existing ones in different ways. Additionally, mystery is often present as the characters try to figure out the secrets of the new world they've entered.
One possible title could be 'The Crossover of Fate: Cu Chulainn's New Adventure'.
Well, a simple yet effective title could be 'The Tangled - Guardians Crossover Fanfic'. However, if you want something more descriptive, you could go for 'Tangled's Rapunzel in the Realm of the Guardians: A Fanfiction'. This title clearly shows the main character from Tangled, Rapunzel, entering the world of the Guardians, which is a great concept for a crossover fanfiction. You could also consider 'Guardians' Guide to Tangled: A Crossover Fanfiction', which implies that the Guardians are somehow involved in the Tangled universe in the story.
Another theme is cultural exchange. The RWBY world has its own set of values, cultures, and technologies, and Touhou has its own very distinct world of youkai and magic. Fanfictions often show how the characters from each world learn about and adapt to the other's culture. For example, a RWBY character might be fascinated by the way magic is used in Touhou. It can be really interesting to see how they incorporate elements of each other's worlds into their own understanding. And this can also lead to some character growth as they expand their perspectives.
Well, in Touhou Project crossover fanfiction, a cool concept is the combination of Touhou with modern-day settings like a big city. Imagine Marisa Kirisame flying her broomstick over skyscrapers. Also, crossovers with other video game worlds are great. For instance, Touhou characters in the world of Super Mario, where they might have to deal with Mario - style obstacles. It's all about mixing the familiar Touhou elements with something new and unexpected.
The unique elements also include the mixing of the two different types of enemies. You have the grotesque and often very large enemies from Dark Souls like the demons and dragons, and then you have the more fantastical and sometimes cute enemies from Touhou. In the crossover fanfiction, seeing how the characters from each side deal with the other side's enemies is really engaging. For example, a Dark Souls knight trying to figure out how to fight a mischievous youkai from Touhou. And the way the stories play with the concepts of souls in Dark Souls and the spiritual energy in Touhou is also quite special.
One popular 'touhou crossover fanfiction' is the one that crosses over with 'Pokemon'. It's really cool to see the characters from Touhou in the Pokemon world, with unique storylines like Reimu becoming a Pokemon trainer. Another great one is the crossover with 'Final Fantasy'. The magic systems of both get intertwined in interesting ways.