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.
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.
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.
One plot idea could be Naruto being banished to the world of One Piece. He could join the Straw Hat Pirates and use his ninja skills in new and exciting ways. For example, his shadow clone jutsu could be used for various tasks on the ship. Another idea is that he gets banished to the Bleach universe. He might learn about Soul Reapers and Hollows, and his chakra could be seen as a unique form of spiritual energy there. Maybe he could even train with the Soul Society to become stronger.
A really interesting plot could be that Naruto is banished due to political reasons in Konoha. He's married to Ino (just as an example). Then, through some sort of magic or strange occurrence related to the crossover element, they are transported to a world full of magic users. Naruto has to learn how to use magic while still holding on to his ninja skills. His marriage to Ino is both a source of strength and a challenge as they face the new and often dangerous situations in this foreign land.
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.