Love is a very common theme. Often, there's a romantic relationship between Ichigo and a vampire character, which is full of passion and conflict. Another common theme is power. Ichigo's new - found vampire powers and how he uses or struggles with them is frequently explored.
A possible plot could be that a powerful artifact in the Rosario Vampire world is being targeted by both Hollows and some rogue vampires. Ichigo and Tsukune find themselves on a quest to protect it. Along the way, they encounter various challenges and meet different characters from both universes. The cultural shock between Ichigo's world of Soul Reapers and Tsukune's world of monsters and humans could also add some comedic and interesting elements to the story.
If Ichigo becomes a vampire in fanfiction, he might gain enhanced physical abilities like super strength and speed. His appearance could also change, perhaps with paler skin and redder eyes. His relationship with his friends might be strained as they might fear or be suspicious of his new vampiric nature.
One common plotline is the transformation. For example, Ichigo might be turned into a vampire and has to deal with his new powers and instincts while still maintaining his relationship with Rukia. This can lead to a lot of internal conflict as he struggles between his human side and his new vampiric nature.
The theme of self - discovery is also prevalent. Ichigo has to figure out what it means to be a vampire lord, his place in the vampire world, and how to balance his new life with his old one. He often goes through a journey of self - exploration and growth in these fanfictions.
The Soul Reapers could be antagonists if they view Ichigo's vampirism as a threat to the balance between the living and the dead. They might try to hunt him down to eliminate this new, unregulated force.
Begin with a prologue that sets up the situation. Maybe there are strange energy readings in both the Bleach world and the Rosario Vampire world that are somehow linked. Ichigo is sent to investigate in his world, and as he gets closer to the source, he is transported to the Rosario Vampire world. Once there, he is greeted by the chaos of the Yokai Academy. You can then start to introduce the different characters and their reactions to Ichigo's sudden appearance. This will slowly build the foundation for the rest of the story.
I really like 'Ichigo: The Vampire Slayer Turned Vampire'. This fanfiction flips the script on Ichigo's role. He goes from being a slayer to becoming what he once hunted. It has a lot of twists and turns in the story. Additionally, 'Vampire Ichigo's Redemption' is a story that follows Ichigo as he tries to atone for his past actions as a vampire in a world full of different supernatural beings.
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.
Well, obviously Ichigo and Uryu are the main characters. Ichigo is a powerful fighter with a strong sense of justice. He has his own set of powers that he brings to the story.