Typically, his old team could include Pikachu, of course. Then there might be Charizard, Bulbasaur, Squirtle, and others like Greninja. These Pokémon have been with Ash through different adventures in various regions.
They might have adventures exploring new areas in Alola that they didn't get to before. For example, finding hidden caves filled with rare Pokémon or uncovering ancient secrets about the region.
His old team would be Sasuke, Sakura, and Kakashi. They are the main characters associated with Naruto's early team experiences. In the context of this fanfic where they die, it's likely to have a huge impact on Naruto and the overall story.
Sure. 'Ash's Alola Pokemon Team' is a fanfic that centers around exactly that. It shows how Ash assembled his team in Alola, the training they went through together, and their unique bonds. Another one is 'The Alola - bound Pokemon of Ash' which gives insights into the individual personalities of his Alola Pokemon and how they work together as a team.
It could be Pikachu. Since Pikachu has been with Ash from the very beginning and is his most iconic Pokémon. It's very likely that in Sinnoh fanfiction, Pikachu would be frequently used as it has a strong bond with Ash and is also very powerful.
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.
In fanfiction, Ash's secret Pokémon team could consist of some rare and powerful Pokémon. Maybe he has a shiny Charizard that he's been training in secret. It could have unique moves and a super strong bond with Ash, making it a formidable addition to his team.
Perhaps there is a threat to Sinnoh that only Ash's old Pokémon can help solve. Some kind of ancient evil that has knowledge of Ash's past Pokémon battles. Ash and his old Pokémon then have to work together to defeat this threat. They might need to combine their old strategies with new ones they learn in Sinnoh to succeed. And along the way, Ash might meet some of his old friends who are also in Sinnoh and they help him in his mission.
Yes, in some Team Rocket fanfictions, Ash can be depicted in various ways that might involve him having some sort of connection to Pokémon in a different light. Maybe he could be experimented on by Team Rocket to gain Pokémon - like abilities, or be transformed into a Pokémon - like being in a very creative and out - of - the - box fanfiction concept.
Ash has always been a defender of Pokemon. If he found out about Team Plasma's experiments, he would first try to understand the full scope of what they were doing. Then, he would use his experience and his Pokemon team's skills to fight against Team Plasma. He might also try to convince some of the members of Team Plasma that what they are doing is wrong, as he has a way of seeing the good in people. Ash would be very determined and would not give up until he put an end to the experiments.
When Ash catches Greninja, it might change the dynamics within his team. Greninja could be a bit of a loner at first, not really fitting in with the rest of the Pokémon. But over time, through shared battles and experiences, it could learn to work well with the others and even inspire them to improve.