A new and powerful rogue ninja could be the antagonist. This ninja could have a grudge against Konoha and sees Naruto as a symbol of Konoha's strength. So he tries to take Naruto out, but Gaara steps in. This new character could bring fresh and unexpected challenges to the story.
It could be Orochimaru. He's always been after Naruto for his special powers. He might set a trap for Naruto, but Gaara intervenes and saves him. Orochimaru's motives of immortality and his dark jutsu make him a great antagonist for this kind of story.
The Akatsuki could also be the antagonists. They might target Naruto for the Nine - Tails within him. If they manage to corner Naruto, Gaara could come to his rescue. The Akatsuki members with their diverse and powerful abilities would pose a serious threat in the fanfiction.
The Akatsuki could also be the main antagonists. They are after Jinchuriki, and Naruto and Gaara being together would be a big target for them. They'd likely try all sorts of sneaky tactics to capture them, which would put Naruto and Gaara's friendship and skills to the test.
Well, it could be that Naruto is trapped in a genjutsu and is on the verge of losing his mind. Gaara, with his unique ability to sense Naruto's distress, breaks into the genjutsu world and saves Naruto. After that, they could have a deep conversation about their past and how they've both grown.
It could be Orochimaru. He might manipulate Gaara into kidnapping Sakura. Orochimaru could have his own devious plans for Sakura, like using her medical knowledge for his experiments. So, he uses Gaara's power and his somewhat unstable mental state at times to achieve his goal. Then Sakura and Gaara would have to team up to fight against Orochimaru.
Maybe it's a new villain created specifically for the fanfiction. This villain could have powers that are extremely dangerous to Naruto, like the ability to drain his chakra completely. So the Shinigami has to step in to save Naruto from this new and powerful foe.
Maybe a rogue ninja from Gaara's world who somehow follows him into the DxD highschool. This ninja could have a vendetta against Gaara and use the chaos of the DxD world to try and take him down. They could also team up with some of the devils who dislike Gaara's presence to create more trouble for him.
Another possibility is a group of Death Eaters who have broken away from Voldemort's control and formed their own faction. They see Harry as a symbol of the old regime that they want to overthrow. They target Harry, but his parents, with their knowledge of the old ways and their love for Harry, are able to save him from this group's attacks.
It might be a magical creature that has been corrupted by dark magic. For example, a hippogriff that has been turned evil. It attacks Harry when he's out exploring. Lockhart, who has some knowledge of magical creatures, tries to calm it down at first but when that fails, he has to use more forceful magic to save Harry from the enraged hippogriff.
It could be All For One. He's a powerful and menacing villain in the My Hero Academia universe. Since Kurogiri is saving Deku, All For One might be trying to target Deku for his own purposes, like stealing his quirk or using him as a pawn in his grand scheme to take over the hero society.
Maybe a new character could be introduced as the main antagonist. This character could be from a hidden village that has been secretly observing Naruto's growth. They realize his potential as a weapon and plan to steal his power or use him to start a war against other villages.
Krampus himself could be the main antagonist. Since he is a figure known for punishing the naughty, his actions in the Naruto world might be seen as unjust. He could be going after ninjas without really understanding their motives, causing a lot of trouble for the main characters.