Lucario is also a popular choice to be paired with Pikachu in crossovers. Lucario has its own unique abilities related to sensing and combat. When combined with Pikachu's electric attacks, they can form a formidable team. The contrast between Lucario's more serious and stoic nature and Pikachu's cute and energetic personality also adds depth to the story.
One interesting concept could be Pikachu crossing over with other Pokémon in unexpected ways. For example, a crossover with a water - type Pokémon like Squirtle might lead to Pikachu having to adapt to an aquatic environment, which could bring about new adventures and challenges.
One idea could be that Detective Pikachu teams up with a character from another universe, like Harry Potter. Pikachu, with his detective skills, could help solve a mystery in Hogwarts. Maybe a series of magical creatures are disappearing, and Pikachu's ability to sense things that others can't would be crucial in finding the culprit.
Obviously, the main characters are usually Pikachu. But sometimes, there could be other Pokémon that play important supporting roles. For instance, a Pichu could be a sidekick or a younger Pokémon that the Pikachus are protecting.
One interesting plot could be two Pikachus from different regions meeting and learning about each other's unique electric attacks. For example, one Pikachu might know a special move that the other has never seen before. They could then teach each other and become stronger together.
I'm not sure if there is a specific 'ash no pikachu fanfic' out there. There are countless fanfics in the Pokémon fandom, but this particular one might be hard to find. You could try searching on dedicated fanfic websites like FanFiction.net or Archive of Our Own and use the relevant keywords to see if it exists.
The third challenge is the fan reaction. Different fans of the original works might have different expectations. Some fans of Detective Pikachu might not like how their favorite character is portrayed in a crossover, and the same goes for fans of the other universe. So, you have to be careful to respect the source materials while also creating something new and exciting in the fanfiction.
The idea of 'Pikachu in a Lord of the Rings setting' is very creative. Pikachu could be on a quest to destroy the One Ring, using its electric powers to fend off orcs and other dark creatures. And 'Pikachu and the Pokémon / Digimon Crossover' is also a great idea. Pikachu could interact with Digimon, compare their powers, and fight together against common threats. This would blend two popular monster - collecting franchises in a really interesting way.
Naruto would probably be really excited. He'd treat Pikachu like a new friend and teammate. He might try to learn all about Pikachu's powers and how it can help in battles. For example, he'd be eager to see Pikachu use Thunderbolt in a fight against an enemy ninja.