One common theme could be the clash of cultures. Naruto's ninja culture and the Pokemon trainer culture are very different. So exploring how characters adapt to this new culture they're exposed to is a theme.
I'm not sure of specific 'naruto banished pokemon crossover fanfiction' titles off the top of my head. But you can try searching on fanfiction websites like FanFiction.net or Archive of Our Own. They usually have a wide variety of crossover fanfictions.
Kakashi would probably be very cautious. He would understand Naruto's reasons for banishing the Pokemon but would also be aware of the potential risks. He might try to counsel Naruto and make sure he has thought through all the implications. He could also be in charge of monitoring the situation to see if there are any unforeseen consequences.
One common theme is redemption. Naruto, being banished, often has to prove himself all over again in the new crossovers, seeking redemption for whatever led to his banishment in the first place.
If Naruto is banished to the My Hero Academia universe, Deku could be his ally. Deku, with his determination to be a hero, would understand Naruto's drive to be recognized. They could share their experiences of being underestimated at first. Also, in a crossover with the Inuyasha world, Kagome could be an ally. She has a kind heart and could help Naruto understand the new world he's in. Her spiritual powers might also interact in an interesting way with Naruto's chakra.
In the Naruto Banished Pokemon Fanfiction, it could be that a powerful villain has the ability to control Pokemon and is using them to attack Naruto's world. Naruto, having no other choice, decides to banish the Pokemon to a different dimension to stop the attacks. However, later he discovers a way to break the villain's control over the Pokemon and has to go on a journey to bring them back and form an alliance with them to defeat the real enemy.
One idea could be that Greninja, being a ninja - like Pokémon, somehow gets transported to the Naruto universe. It could team up with Naruto and use its water - based attacks in combination with Naruto's Rasengan. Maybe Greninja's ability to blend into the shadows could be used in stealth missions in the Hidden Leaf Village.
Another common character might be Superman. If it's a crossover with the DC universe, Superman's sense of justice and his acceptance in his new world (Earth) could contrast with Naruto's banishment. Naruto could learn a lot from Superman about using his powers for good and also about being a symbol of hope, which could inspire him to find his own way to be a positive force even after being banished from his clan.
One popular 'Naruto banished crossover fanfiction' is 'Naruto in Another World'. In this fic, Naruto is banished from Konoha and ends up in a different universe where he has to adapt to new powers and challenges. It's a really exciting read.
To write a great Naruto banished married crossover fanfiction, start with a strong premise. Decide early on why Naruto is banished and who he is married to. Let's say he's married to Temari. The banishment could be because of a prophecy that some in Konoha misinterpret. For the crossover, if it's with a world of mythical creatures, research those creatures. Develop the relationship between Naruto and Temari. Maybe they have different views on how to deal with the new world they are in. Write engaging dialogue that shows their personalities and how they grow as a couple in this new and difficult situation.