Madara could also be the main antagonist in a different - turn 'naruto return to past fanfic'. If Naruto changes things in the past, it might disrupt Madara's original plans. Madara, being a powerful and ambitious character, would not take this lying down. He could become more aggressive in his pursuit of the Eye of the Moon Plan and Naruto would have to face him in a new set of circumstances, making for a very different and exciting story.
It could be the Akatsuki. Since Naruto is now following a more Sasuke - like path, he might attract their attention more. They might see him as a potential threat or someone they can manipulate, just like they did with Sasuke.
It could be a new villain who has been plotting against Konoha while Naruto was away.
It could be Voldemort who somehow crosses over into the Naruto world. He would be a formidable antagonist as he has powerful dark magic. He might team up with some of the villains in the Naruto world like Orochimaru, and together they could plot to take over both the wizarding world and the ninja world. Naruto and Harry would then have to work together to stop them.
Perhaps an old enemy of Itachi's from his past who has also reincarnated or found a way to exist in the modern day. This antagonist would try to use Naruto's kids as a way to get revenge on Itachi, seeing them as a weak point in Itachi's new - found role as a protector.
A powerful rogue ninja who has mastered a forbidden jutsu that gives him the ability to control minds could be a great main antagonist. He could use this power to turn Naruto's friends against him, creating a lot of internal conflict within the group.
It could be a former member of Akatsuki who has a personal vendetta against Naruto. Maybe they escaped from prison and now they want to take revenge on Naruto for all the things he did to Akatsuki. They would be a very powerful and dangerous antagonist as they possess knowledge of many dark jutsus.
It could be Orochimaru. He's always been interested in power and forbidden jutsu. In this fanfic, he might discover the power of Titan shifting and try to use it for his own gain. He could experiment on people to create more Titan shifters under his control, and Naruto would have to stop him from creating an army of these powerful beings.
A new character could be introduced as the main antagonist. Maybe a powerful ninja from another village who has infiltrated Konoha's politics. This character could be the one behind the idea of forcing Naruto back, with the intention of using Naruto as a pawn in a larger scheme against Konoha.
It could be a rogue ninja who took advantage of the chaos during the war. This ninja might have been gathering power in the shadows and now sees Naruto as a threat to his plans.
It could be a rogue ninja from his own village who wants to use Naruto's unique position (being from a different village but having ties to the Leaf through his adventures) to start chaos. This ninja might be jealous of Naruto's popularity and abilities, so he tries to frame Naruto for various crimes to turn the different villages against him.