Well, it could be about Ichigo teaching Izuku some combat skills. Since Ichigo is a powerful Soul Reaper, he might show Izuku how to better use his physical strength and speed in battle.
It's hard to say. Ichigo has his Soul Reaper powers which are incredibly strong. He can cut through almost anything with his Zanpakuto. But Izuku has One For All which gives him superhuman strength, speed, and the ability to jump really high. So it depends on the situation.
In lemon fanfiction, Ichigo Kurosaki might be depicted with a different personality. Sometimes, authors explore his more emotional and vulnerable sides that might not be as prominent in the original work. For example, he could be shown in a romantic relationship where his protective nature is emphasized in a new way.
One appeal is that it gives fans a chance to see Ichigo in new and unexpected situations. Since lemon fanfiction often involves romantic or steamy elements, it can show a different side of his character that we don't usually get to see in the main series.
One idea could be a crossover where Ichigo and Naruto are transported to a strange new world together. They have to combine their skills, like Ichigo's Zanpakuto abilities and Naruto's ninjutsu, to find a way back home.
Another fanfiction could be a story where Sakura, for some reason, is transported to the world of Bleach. She enrolls in the Soul Reaper academy, where she meets Ichigo. They start off as rivals, constantly competing in combat training and academics. But as time passes, they realize they have a lot in common and end up becoming good friends and allies.
One idea could be a story where Ichigo, with his strong sense of justice, has to confront the manifestations of the Seven Deadly Sins in the Soul Society. For example, he might encounter a Hollow that represents Greed, which is hoarding spiritual energy. He could use his Zanpakuto skills to defeat it and learn something about his own potential for that sin. Another could be a crossover where the characters from Seven Deadly Sins end up in the Bleach universe and Ichigo has to help them find their way back, all while dealing with the chaos they bring. It could be interesting to see how Ichigo's power compares to the abilities of the Seven Deadly Sins characters.
There's 'Soulmate's Journey' which is a really popular one. It delves into the idea that they are soulmates in a more spiritual and emotional sense. It has some really touching moments that show their love and loyalty to each other. It also includes some great action scenes that are true to the Bleach universe.
A fanfiction could focus on Ichigo attending the school in Highschool DxD. His presence would surely disrupt the normal balance. He might not be interested in the whole 'devil' and 'angel' thing at first, but gradually get involved. He could also introduce some of his Soul Reaper training methods to the students there, which would be very different from what they're used to.
First, you need to understand the characters well. For Ichigo, know his personality traits like his determination and his sense of justice. For the Highschool DxD characters, understand their hierarchies and their goals. Then, come up with a unique plot. It could be a battle - oriented story where Ichigo and the DxD characters fight a powerful enemy together. Or it could be a more slice - of - life story where Ichigo adapts to the strange new world of DxD.