Perhaps there's a world where vampires and Soul Reapers co - exist, and Ichigo and Orihime get caught in the middle of a power struggle between the two groups. Orihime's powers might be the key to peace, and Ichigo is determined to protect her.
A plot could involve Dark Ichigo and Orihime getting trapped in an alternate dimension. In this dimension, they have to rely on each other to survive. Dark Ichigo has his brute strength, but Orihime has her powers that could be useful in different ways. Their relationship would develop as they face various challenges together in this strange new world.
One idea could be exploring how Ichigo and Orihime deal with the challenges of raising a Quincy - Hollow hybrid child. Since Ichigo has both Hollow and Shinigami powers and Orihime has her own unique abilities, their child might inherit a complex set of powers.
Maybe the wedding is a ruse to draw out an enemy. Ichigo and Orihime pretend to be getting married, but in reality, they are setting a trap. However, as they go through the motions of the wedding preparations, they start to realize how much they actually do want to get married for real. So, in the end, after defeating the enemy, they decide to have a real wedding. It could also be that Orihime's family has some ancient traditions related to marriage that Ichigo has to go through, which could lead to some really funny and heartwarming moments.
One idea could be a post - war story where Ichigo and Orihime finally tie the knot. They could have a traditional Japanese wedding with all their friends and family from the Soul Society and the human world in attendance. Another plot could be a story where they face some last - minute challenges before the wedding, like an old enemy trying to disrupt their happiness. Maybe Orihime has doubts about whether she's truly worthy of Ichigo, and they have to work through those insecurities together.
One idea could be that Ichigo, Rukia, and Orihime discover a strange lemon - shaped spiritual artifact. They have to figure out its powers and how it relates to the Soul Society. Maybe it has the power to enhance their spiritual abilities, but also comes with a strange side - effect that changes their relationships in unexpected ways.
One idea could be Ichigo struggling to control his new vampire instincts while still trying to protect his friends. He might have to hide his vampiric nature from his family, which adds a layer of secrecy and tension.
One idea could be exploring how they adjust to the news. Maybe Ichigo becomes overly protective and Orihime has to calm him down. Another could be their friends' reactions. Chad might be really supportive while Uryu is a bit more reserved at first but then warms up.
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.
I'm not sure about a specific 'Ichigo and Orihime lemon fanfiction' title off - hand. But you can often find great ones on fanfiction websites. Just search for 'Ichigo Orihime relationship fanfiction' and look through the results. You might have to sift through a bit to find the ones that are well - written and suit your taste.