Neliel could be a great choice. She has a more gentle and kind nature compared to many other Arrancars. Her unique appearance and backstory would make for an interesting dynamic with Uryu. She could potentially teach Uryu about the different aspects of Arrancar existence that he may not be aware of.
One idea could be that Uryu, with his Quincy skills, is forced to team up with an Arrancar to fight a common enemy that threatens both their worlds. This could lead to a slow - building trust and friendship between them.
A Hogwarts professor who has knowledge of both the wizarding world and some dark secrets related to other dimensions. This professor could play a key role in dealing with the Arrancar, whether it's trying to communicate with it or stop it from causing harm.
Luffy from One Piece could also work well. Luffy's carefree and optimistic nature against Gajeel's more serious and gruff personality would create an interesting dynamic. Maybe they're on a quest to find a powerful treasure in a new world created by the crossover.
A werewolf could be an interesting character to pair with a vampire wife. Their natural enmity in most lore could create a lot of tension and drama in the story. But also, they could find common ground and fall in love, defying the odds.
Superman could be paired with Obito in a crossover. Superman's pure - heartedness and incredible powers could influence Obito. They could work together to protect a city under threat. Obito might be in awe of Superman's abilities at first but then find ways to contribute with his ninja techniques.
In a crossover with the Fate series, Saber could be paired with Toguro. Saber's sense of honor and her noble fighting style would clash with Toguro's brute force and underhanded tactics. It would create a really interesting dynamic, whether they are on opposing sides or forced to work together for some greater purpose.
In a crossover with the Naruto universe, Sasuke could be a great character to pair with Illumi. Both are somewhat dark - natured and highly skilled. They could start off as enemies, with Illumi seeing Sasuke as a target or a rival in power, but then gradually develop an understanding or a strange alliance as they realize they have common goals in terms of seeking power and revenge in their own ways.
One great arrancar ichigo fanfiction is 'The Hybrid's Path'. It explores an alternate universe where Ichigo has more complex interactions with the arrancars. Another is 'Ichigo's Arrancar Ally' which focuses on an unlikely alliance between Ichigo and an arrancar against a common enemy. And 'Arrancar - Infused Ichigo' shows Ichigo with some unique arrancar - like powers.
There could be some original Arrancar characters created specifically for the fanfiction. For example, a young and curious Arrancar who is new to the ways of her kind. She meets Naruto and is immediately fascinated by his different appearance and abilities. And then there's a more experienced Arrancar who has seen many battles. She initially sees Naruto as a threat but then realizes his good intentions and slowly falls for him, becoming part of the harem.
Rukia could be one of the characters in the harem. She has a long - standing relationship with Ichigo and her own strong spiritual powers. Orihime could also be part of it. Her kind - hearted nature and unique abilities would add depth to the harem dynamic. Another potential character could be Neliel. As an arrancar herself, she could have an interesting relationship with arrancar Ichigo.