It could be a new character who has ice - based powers and is out to take over the ninja world. This antagonist might see Naruto as a threat to his plans because of Naruto's growing power and popularity.
Maybe an old enemy of Naruto's who has somehow gained ice - like abilities and uses them to exact revenge. For example, Orochimaru could have found a way to transform himself or his minions into ice - based threats, specifically targeting Naruto to stop him from achieving his goals.
Perhaps it could be a powerful ice spirit that has been awakened by some dark jutsu. This ice spirit has no concept of good or evil but is causing destruction as it spreads ice everywhere. Naruto and his friends would have to find a way to either seal it or make it understand the harm it's causing. In this case, the ice spirit would be the main antagonist as it opposes the well - being of the villages.
The main antagonist could be a powerful Ice Age beast that has been corrupted by dark chakra. This beast might see Naruto and his friends as invaders in its territory and attacks them without mercy. It could have abilities like freezing its opponents with a single breath or being able to move at incredible speeds on the icy terrain.
Perhaps there is no traditional 'big bad' antagonist. Instead, the antagonist could be the overall neglect and apathy of the village itself. It's like the village's indifference towards Naruto when he's sick with a cold is the force he has to fight against. He has to break through this wall of neglect to get the care and respect he deserves.
Sasuke could be the main antagonist. In high school, he might be jealous of Naruto's popularity or newfound skills. He could try to undermine Naruto at every turn, just like their complex relationship in the original Naruto series.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
Maybe it's a group of rogue demigods who have turned to the dark side. They found a way to manipulate the power of ice and are using it to try and take over Camp Half - Blood. Their leader could be a demigod with ice - related powers who has gone mad with power.
It could be an evil ice dragon that has gone mad with power. This dragon might be hoarding all the ice magic in the land, causing problems for the people and other magical creatures. Lucy would then have to face this powerful foe to restore balance.
Maybe Hawk Moth himself decides to target the ice - skating community. He could akumatize a skating judge who then gives unfair scores to the skaters. Ladybug and Chat Noir would have to deal with this unjust situation and stop the akumatized judge from causing more chaos in the skating world.
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.