Naruto's never - give - up attitude would be a huge influence. Riser, in the original story, sometimes took the easy way out. But with Naruto's personality, he would be more likely to face challenges head - on. For instance, in battles or in dealing with political situations within the DxD world. Also, Naruto's loyalty to his friends would make Riser more trustworthy in the eyes of others.
Naruto could win by using his Sage Mode. In Sage Mode, his strength, speed, and perception are enhanced. He could easily dodge Riser's attacks and counter with powerful Sage - enhanced jutsu. His ability to sense chakra would also help him predict Riser's moves.
Maybe in the fanfiction, Naruto is reincarnated as Riser Phenex in the Naruto world. He has to deal with the memories and instincts of Riser while also trying to maintain his identity as Naruto. He could use Riser's abilities, like his fire - based powers and his unique magic, to fight against new threats in the Naruto universe and also to protect his friends and village.
In a Naruto - reborn - as - Riser - Phenex fanfic, Naruto could introduce new fighting styles to the DxD universe. His taijutsu skills combined with Riser's magic could create a whole new combat system. Additionally, his experience with the Nine - Tails could translate into something unique in this new world. Maybe he could have a sort of internal 'demon' like power within Riser's body that he has to learn to control, just like he did with Kurama.
You can try searching on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics from various fandoms, and there's a good chance you'll find some 'riser phenex fanfic' there.
One good 'riser phenex fanfic' could be 'Riser's Redemption'. In this fic, Riser might go through a character arc where he overcomes his flaws and becomes a more respected figure in the supernatural world. It could show his growth from being a somewhat self - centered character to one who cares about others and uses his powers for good.
The plot would take on a more action - oriented and heroic tone. Naruto is a hero at heart, always looking to protect the weak and fight for what's right. In contrast to Riser, he would actively seek out problems to solve and villains to defeat. His presence would also attract different kinds of antagonists, those who oppose his ideals of peace and justice, which would create a whole new set of storylines.
Naruto might become more calculating. Urahara is a very strategic person, and under his training, Naruto could start thinking more before acting. For example, instead of just rushing into a fight, he'd analyze the situation first.
Naruto might become more confident, bordering on cocky at times. The Super Saiyan 4 transformation comes with a lot of power, and with that power, he could start to believe he's invincible. But deep down, his core values of protecting his friends and the village would still remain. He might also become a bit more impatient, as the power coursing through him could make him eager to jump into battles quickly.
I'm not sure specifically about a 'naruto is riser phenex' fanfiction. You could try searching on fanfiction - related websites like FanFiction.net or Archive of Our Own. They have a vast collection of fanfictions, and you might be able to find one that fits this concept by using their search functions.
Naruto's personality could become more brooding. Having the power of the Death Note might make him more aware of the darker side of power. He might not be as open and friendly as he usually is, since he has this secret and rather sinister tool at his disposal.