Perhaps some of the more power - hungry ninja leaders who fear the new power that Naruto - Kaguya represents. They might think that this new entity will disrupt the power balance they've established. So, they could form an alliance against the merged being, making them the antagonists in the story.
It could be the remaining Otsutsuki clan members. Since Naruto has merged with Kaguya, they might see this as a threat to their plans for the ninja world or a perversion of their bloodline. They could try to either eliminate the merged Naruto - Kaguya or control it for their own purposes.
There could be ancient evils that were sealed away long ago and are now awakened by the powerful chakra signature of the merged Naruto - Kaguya. These evils, having been dormant for so long, see this new power as a way to break free completely and cause havoc, thus becoming the main antagonists as they oppose the merged entity's attempts to maintain peace in the ninja world.
Chapter 562, "So You Are Not First, But Last?!", is the clearest early answer because it turns Team's situation into a concrete plot event. The chapter turns that into something concrete by moving from After they finished preparing the fish, Guo Xing and Luo Xuan got...
Chapter 810, "I'm Waiting for Death", is the clearest early answer because the violence around Brother becomes final instead of hypothetical. It opens with Jiang Buchen was about to go crazy from hunger. and closes on Alright, he's satisfied even if he dies after eating such tasty food..
Chapter 561, "How Can This Young Master Use His Hands to Kill Fish?", is the clearest early answer because the violence around Luo Xuan becomes final instead of hypothetical. The chapter gives readers a real answer by opening on She had saved Luo Xuan that afternoon, and he already owed her one.
Chapter 460, "Saving People", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from [HAHAHAHAHAHAHA, Jiang Yan is impersonating me.] [At this moment, I... to Dare to attack me in hiding?!.
Chapter 513, "A Public Stage", is the clearest early answer because it turns Jiang Mingyu's situation into a concrete plot event. Early on, Everyone was sharing their experiences in a few words, the banter.... By the end, It's Shiguang's group!" "Performing right after Xi Yang, that's....
Chapter 497, "Carefully Raised a White-eyed Wolf", is the clearest early answer because it turns Team's situation into a concrete plot event. A useful checkpoint is the way the scene starts with The setting sun packed up its last rays and tossed them into the...
Chapter 409, "I'm Feeling Lucky!", is the clearest early answer because it turns Yu Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Jiang Yan was still arguing with Yu Li with his hands on his hips,...
Chapter 466, "Are There Really Wild Boars?!", is the clearest early answer because it turns Shang Shuyue's situation into a concrete plot event. It opens with The lady sure acts like a lady, even in 'Wilderness Survival' she...
Chapter 775, "Live Stream the Day After Tomorrow!", is the clearest early answer because it turns Shen Lan's situation into a concrete plot event. Early on, After meeting Jiang Li, Jiang Shixu became much happier.. By the end, There were some conflicts between the spouses even though they had....
Chapter 505, ""Postpartum Care for Sows", is the clearest early answer because it turns Shang Shaobai's situation into a concrete plot event. Early on, Zhao Lang laughed, but his eyes became considerably clearer, "Is....