Perhaps a vampire scientist who is conducting unethical experiments on both humans and vampires. Ichigo gets in his way when he discovers what the scientist is doing. This antagonist could be using his scientific knowledge to create powerful and dangerous hybrids. He believes that he is creating a new and better vampire - human world order, but his methods are causing a lot of harm, and Ichigo is determined to stop him.
There could also be a powerful vampire hunter. This person might have a personal vendetta against vampires in general or specifically against Ichigo. They could use all kinds of weapons and tactics to try and kill him.
Well, a plot could involve Ichigo being turned into a vampire against his will. He then has to deal with the internal conflict of his human morals versus his new vampiric instincts. He might find that some of his friends are scared of him, while others are determined to help him find a way to reverse the transformation. And there could be a love interest who accepts him despite his new nature, adding a romantic sub - plot to the story.
One idea could be that Ichigo has to hide his vampire nature from his friends at first. He struggles with the new instincts and powers, like enhanced speed and strength. Maybe he has to resist the urge to drink blood from his loved ones.
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.
A third idea is that Ichigo is a secret beta - tester for a new expansion of SAO that somehow links to the Soul Society. He has to keep his identity a secret while also using his skills to protect the other players from new threats that are emerging due to the strange connection between the two worlds.
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.