Madara Uchiha could be a great antagonist. He already has powerful jutsu and a thirst for power. If he gets wind of the Infinity Gauntlet, he would surely try to take it for himself to achieve his goal of creating a world under his control.
Sasuke could potentially be the main antagonist in this fanfiction. If he believes that the power of the Infinity Gauntlet can help him avenge his clan in a more decisive way or gain enough power to reshape the ninja world according to his vision, he might turn against Naruto to get it.
It could be Doctor Octopus. He's always been one of Spider - Man's toughest foes. With Spider - Man having the Infinity Gauntlet, Doctor Octopus would be extremely jealous and want it for himself to achieve his own evil goals, like taking over the city or the world.
Well, in this fanfiction, perhaps the Infinity Gauntlet is brought into the Naruto world by a mysterious portal. Naruto, being the curious and brave ninja he is, picks it up. He might initially struggle to control the stones' powers as they are different from chakra. But as he learns, he could use it to unite the shinobi clans in a way never seen before, like using the Time Stone to show the clans the history of their pointless feuds and convincing them to work together for a better future.
A likely main villain could be Kronos. He's always been after power and the Infinity Gauntlet would be the ultimate tool for him to finally overthrow the Olympians and rule over everything. He could sense the power of the gauntlet and do whatever it takes to get his hands on it.
In this fanfiction scenario, Naruto finding the Infinity Gauntlet could lead to some interesting battles. He could use it against his toughest enemies like Madara or Kaguya. But he'd also have to deal with the fact that the Infinity Gauntlet comes from a different universe with different rules. He might struggle to control all the power it holds at once.
One possible plot could be that Naruto uses the Infinity Gauntlet to bring peace to the ninja world. He might snap his fingers and make all the hatred and grudges disappear. With the power of the Infinity Stones, he could also restore all the lives lost in the previous wars, like bringing back his parents and all the fallen shinobi. This would completely change the power dynamics in the Naruto universe and lead to a new era of unity among the different villages.
Well, in this fanfiction, the Joker could find out about Batman having the Infinity Gauntlet. He would then try to steal it, thinking he could create the most chaotic and hilarious situations. Batman would have to fight off the Joker while also dealing with the power of the gauntlet. Maybe the gauntlet gives Batman the ability to see into the Joker's mind and understand his madness better.
In this fanfiction, his biggest threat could be Sasuke. Sasuke has always been Naruto's rival and he might see Naruto with the Infinity Gauntlet as a threat to his own goals. He could try to take the gauntlet from Naruto to either use it for his own means or to stop Naruto from misusing it. Another potential threat could be Orochimaru. He is always after power and the gauntlet's power would be too tempting for him to resist. He would surely plot to steal it from Naruto.
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.
Naruto would likely see it as a new way to protect his village and friends. He's always been about that - protecting what he loves. But he'd also be cautious because he knows that power can be dangerous. He might think about how to use it in a way that doesn't corrupt him, like how he was careful with the power of the Nine - Tails.
Thanos is the main character as he is the one who gets the Infinity Gauntlet. Also, heroes like Iron Man, Captain America, Thor, Hulk and others are main characters too as they try to oppose Thanos.