Well, when it comes to combining their abilities in fanfiction, it's all about creativity. Naruto's ability to sense danger using his chakra could be linked with Astolfo's precognitive abilities (if he has any in the fanfic). This way, they could anticipate their enemies' moves even better. Also, Astolfo's charm - based abilities could potentially be used to sway some of Naruto's more stubborn opponents, while Naruto's raw power could back up Astolfo in a fight.
Naruto's ability to use chakra and his powerful jutsu like the Shadow Clone Jutsu could be combined with Hanabi's Hyuga clan techniques. In the Sekirei world, they might use these combined abilities to take on stronger opponents. For example, Naruto could create clones to distract enemies while Hanabi uses her Byakugan to find weak points and strike.
Naruto's ability to create shadow clones could be used in strategic ways in Rias's battles. For example, he could send clones to scout out the enemy while the devils in Rias's harem set up their attacks. His Rasengan could also be combined with the devil's energy - based attacks. Maybe a devil could enhance the power of the Rasengan with their own magic. Also, Naruto's chakra control could help the devils in Rias's group to better control their own internal energy sources, as chakra control is a very refined skill in the 'Naruto' universe.
A lot of the time, Doomguy's indomitable will and his combat experience in the face of demons can be paired with Naruto's ninja skills. Picture Doomguy charging into battle, and Naruto using his wind - style jutsu to enhance Doomguy's attacks, like making his melee strikes even more powerful with gusts of wind.
Naruto has a great deal of chakra, which is like energy in his world. Angewomon could potentially draw on this chakra to enhance her own powers. In return, Angewomon could teach Naruto some of her more spiritual ways of using energy. This exchange of energy - using and knowledge - could lead to new and unexpected abilities for both of them.
Naruto's Nine - Tails chakra has a huge amount of power. If Scathach could somehow tap into that power, it would be a formidable combination. On the other hand, Scathach's knowledge of different forms of combat and magic could help Naruto expand his arsenal. He could learn to incorporate some of her techniques into his own fighting style, creating a unique blend of shinobi and magical abilities.
Naruto's ability to create shadow clones could be used in combination with Garnet's strength. For example, the shadow clones could distract opponents while Garnet delivers a powerful blow. The clones could also help in gathering information in a battle situation, which Garnet could use to strategize her attacks better.
In a Naruto - Homelander fanfiction, their abilities could combine in some very unique ways. Homelander's invulnerability could be used as a shield while Naruto performs some of his more complex and time - consuming jutsus. Naruto's Sage Mode could potentially give Homelander some insights into a different kind of power - up. And Homelander's super - hearing could be used to detect enemies from afar and give Naruto the necessary information to plan his attacks using his strategic ninja skills.
Naruto's ability to use chakra could be combined with Sebastian's demonic powers in interesting ways. For example, Naruto could use his chakra to enhance Sebastian's physical attacks. Or Sebastian could use his knowledge of the supernatural to help Naruto better understand and control his bijuu chakra.
In some fanfictions, Naruto's Rasengan could be combined with Ranamon's water - based attacks. For example, Naruto could create a Rasengan infused with Ranamon's digital water, making it more powerful and having a different effect. Maybe it could freeze enemies on contact.
The characters' abilities can combine in interesting ways. For example, a ninja like Sakura could use her superhuman strength along with a Pokemon's physical attack. So if she teams up with a Machamp, they could deliver some really powerful punches. Naruto's ability to create multiple clones could be used in combination with a Pokemon that has a wide - area attack move. He could send out his clones and then have a Pokemon like Gardevoir use Psychic to attack a large group of enemies all at once.