Here's a Naruto/Bleach challenge. Now, this is a strange idea I had. Basically, there are two scenario's that I had come up with for this.
First is the original thought: Aizen has a daughter in secret Anya (you don't have to use this name, but im just using it so I don't have to refer to her as Aizens daughter all the time), made from his dna, and whoever else, as part of an experiment to see if hollows and soul reapers could have a child together, and how powerful it would be. Now, after Aizen is defeated, his daughter takes the Hogyoku, and escapes to the elemental nations, in her attempt to gain power to do what her father could not. There she finds out about Naruto, and after learning about his lineage and Kyuubi, she decides to take him for herself, as his potential (without her increasing his powers with the Hogyoku, by granting him hollow and/or soul reaper powers) was great. She and Naruto will of course have a master/servent/lovers relationship later on.
Pairing:
Aizens daughter must be with Naruto. -You can name her whatever you want, as well as make her mother whoever you you could even have it so Aizen spliced dna from several powerful people into her.
Other than that, you can make this a harem. You could also make it where Naruto has a bunch of 'mistresses' and Anya is his only official wife, I don't really care.
Genderbends are allowed as well.
Now, you can make it that Anya wanted to find someone compatible with the hogyoku, and it chose Naruto, if you want.
Version 2 of this: Aizen wants so desperately to be freed of Kisuke's kido, that the hogyoku grants the wish, and teleports Aizen to Naruto's world. But, Aizen is not an adult, but is now around naruto's age, or around mid teens.
Pairing:
fem Aizen must be with Naruto. Other than that, you can make the pairing whatever you want.
Genderbends are allowed.
Note-Aizen (or if you want to change the name to something more effeminate) will still have all of her abilities as a soul reaper, but all skills she gained from the Hogyoku will take time to recover, considering the dimensional jump and age reduction (or be removed altogether if you want)
Other than all that, do what you want.