Pikachu is definitely a common character. It's Ash's most loyal partner and is always by his side during his adventures. Serena is also often present. She has her own dreams and goals related to Pokemon and often travels with Ash, providing support and sometimes even competing with him.
A great team could be Greninja, Charizard, Sylveon, Lucario, Tyranitar, and Hawlucha. Greninja is fast and strong. Charizard has high attack power. Sylveon can be a good support. Lucario is a great all - rounder. Tyranitar is bulky and powerful. Hawlucha is quite agile in battles.
One interesting story could be about Ash's journey in the Kalos region in a more in - depth way. For example, his encounters with different Pokemon and trainers that weren't fully explored in the anime. Another might focus on his relationship with his Pokemon in a new light, like how he trains them specifically for Kalos - style battles.
One exciting plotline could be Ash encountering a new and extremely powerful Pokemon that he has to figure out how to catch. Maybe it has unique abilities that no one has ever seen before.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. These platforms have a large collection of fan - created stories, and you can simply search for 'Pokemon Ash X and Y' to find relevant fanfictions.
You can't go wrong with a team of Snorlax, Dragonite, Ninetales, Arcanine, Lapras, and Crobat. Snorlax is a tank with high HP. Dragonite is extremely powerful. Ninetales has some cool fire - type moves. Arcanine is a fast and strong fire - type. Lapras is useful for water - type attacks and can also learn ice - type moves. Crobat is very agile in the air and can deal decent damage.
One idea could be Serena and Ash getting lost in a mysterious forest in the Kalos region. They have to rely on each other's skills and the Pokemon they have with them to find their way out. Along the way, they might discover a new and rare Pokemon species.
Their relationship could develop further through shared adventures. For example, if they team up to rescue a group of endangered Pokemon, they would have to rely on each other's strengths and trust each other completely. This would deepen their bond.
One possible strongest team could be Pikachu, Charizard, Greninja, Lucario, Sceptile, and Infernape. Pikachu has been with Ash since the start and has a strong bond. Charizard is extremely powerful. Greninja has its unique form and great combat skills. Lucario is a very capable fighter. Sceptile was a strong addition to Ash's team in Hoenn. And Infernape showed great power in Sinnoh.
For Ash, Pikachu is always a great choice as it's his most loyal and powerful partner. Alongside that, a strong flying type like Charizard could be useful. For Brock, his Onix would be a staple. And perhaps a water - type like Vaporeon could be good for him to balance his team. Since they are teaming up, having a psychic - type like Alakazam could also be beneficial as it can help in battles and also with communication in tricky situations.