Naruto's spirit could endow Yoruichi's Zanpakuto with some really interesting abilities. For instance, it could have a chakra - coating ability that enhances the cutting power of the Zanpakuto. There could also be a mode where Yoruichi can tap into Naruto's Nine - Tails chakra, which would give her a huge boost in power and a different appearance, like a partial transformation similar to Naruto's Nine - Tails forms. And perhaps there would be an ability to communicate with animals, as Naruto has had some connection with animals in his own story.
The Zanpakuto might gain the ability to sense the emotions and intentions of opponents, much like Naruto can sometimes sense the feelings of others. It could also have a new form of movement, perhaps a super - fast dash inspired by Naruto's speed boosts. Additionally, the spirit of Naruto could give the Zanpakuto the power to resist certain types of spiritual attacks, as Naruto has shown great resistance in his own world.