First, understand Aizen's character traits thoroughly. He's manipulative, intelligent, and power - hungry. In a crossover, you need to show how these traits interact with the new world. For example, if crossing with 'Game of Thrones', his manipulation would be a great asset. He could play the different houses against each other. Second, give him clear goals. If it's a crossover with 'One Piece', his goal could be to find the One Piece and use it to rule the new world. Third, let his actions have consequences. If he makes a wrong move in a world like 'Naruto', the ninja might catch on to his schemes and fight back.
One idea could be a crossover with the 'Harry Potter' universe. Sosuke Aizen could be a dark wizard who infiltrates Hogwarts, using his charm and manipulation skills. He might try to sway students to his side, believing they can help him achieve his goals. Another idea is a crossover with 'The Lord of the Rings'. Aizen could be a powerful being in Middle - earth, perhaps using the One Ring for his own purposes if he got his hands on it. He'd likely deceive those around him to gain more power.
The Soul Reapers would be very confused at first. They are used to Aizen being a certain way, and Naruto's different personality would be a shock. Ichigo might be curious and see Naruto - Aizen as an ally against the common enemies. Orihime could be one of the first to trust Naruto - Aizen because of his kind - hearted nature that she would sense. And the Espadas would probably be on high alert, not knowing what to expect from this new Aizen.
One main feature is the combination of characters from Naruto and Bleach universes. Since Naruto and Aizen are from different shows, it creates a unique blend. Another is the harem element which usually involves multiple characters having romantic or strong emotional ties to Naruto and Aizen.
One possibility is that Naruto's reincarnation as Aizen would lead to a whole new set of battles. He might face off against some of the powerful Hollows or even the more powerful Soul Reapers who see him as a threat. His Naruto - based powers could be a huge advantage in combat. He could use his chakra control to enhance his sword skills or his ability to sense spiritual pressure. And his journey through the Bleach world as Aizen could be about finding a way to bring peace, just like he always aimed for in the Naruto world.
The plot could start with Naruto somehow being transported to Aizen's world. He quickly realizes the danger Aizen poses. Naruto begins to train and adapt to the new environment, learning about reiatsu. As he battles minor foes related to Aizen, he gains knowledge about Aizen's weaknesses. Eventually, when facing Aizen, he uses his ninja skills and new - found understanding of this different power system to land a fatal blow.
Well, in a fanfiction scenario, Naruto might kill Aizen if he manages to use his shadow clones to outwit Aizen's illusions. Naruto could also use his Sage Mode to sense Aizen's reiatsu and counter his attacks. Maybe he combines his Rasengan with some new jutsu he's developed specifically to deal with Aizen's powers.
A popular 'naruto aizen harem fanfiction' is 'Naruto, Aizen and the Harem's Adventure'. It starts with Naruto and Aizen being transported to a new dimension. In this dimension, they encounter a group of characters who gradually form a harem around them. The story is full of action, as Naruto and Aizen use their powers to protect the harem members, and also has some really interesting character development moments.
The storyline could start with Naruto somehow ending up in Aizen's world. At first, he would be confused and on guard. But as he gets to know Aizen, he might see some potential in him. Naruto would try to change Aizen from the inside. Aizen, on the other hand, might see Naruto as a useful tool or an interesting experiment.
In this fanfiction scenario, if Naruto joins Aizen, it could lead to a complete upheaval of the existing power dynamics. Naruto's strong sense of justice would surely clash with Aizen's more self - centered motives. However, they could also find common ground in their pursuit of power, albeit for different reasons. Naruto wants power to protect his friends and village, while Aizen desires power for domination. This conflict and potential for cooperation would make for a really exciting story.