Well, a 'hinata cheats on naruto with kiba fanfic' could have a plot where Hinata gets influenced by some external factors. Maybe she's under a genjutsu that makes her act out of character. Kiba, on the other hand, might be misinterpreting her actions as something more. But overall, it goes against the core relationship values shown in the Naruto series.
In a fanfic like this, it might start with Hinata feeling neglected by Naruto due to his busy Hokage duties. Kiba, being an old friend, is always around to offer support. But this is a very uncharacteristic plot as Hinata is deeply in love with Naruto in the original series. It would likely cause a lot of drama and conflict within the Konoha community.