Another possible antagonist could be a rogue ninja from Hailbel's past. This ninja could have a vendetta against her family or something she did in the past. When he discovers that Hailbel is with Naruto, he decides to take them both on. He might have some unique and dangerous techniques that Naruto and Hailbel have to figure out how to counter.
A powerful vampire - witch hybrid could be a great antagonist. This character could combine the strength and immortality of a vampire with the magical abilities of a witch. They might envy Bella's connection to both the vampire and human worlds, and also be jealous of Chris's powerful magic lineage. Their goal could be to create a world where vampires and witches rule over humans.
Another possible antagonist could be the CCG in the 'Tokyo Ghoul' universe. If they were to somehow cross paths with the 'Death Note' world, they might see the power of the Death Note as a threat and try to take it. This would put them in direct conflict with both Kaneki and Light, as Light wouldn't want to lose his tool for 'justice' and Kaneki wouldn't want the CCG to gain more power to hunt ghouls. So, in this complex crossover situation, the CCG could end up being an antagonist.
Ninjas, of course! They would come to relax after a long day of training or missions. Also, fans of the ninja world from within the story would be there. Maybe even some of the village leaders would stop by to show support for the local business.
Vlad Masters could be a likely antagonist. He might see Kim as a threat to his plans regarding Danny. Since he's always looking for ways to get Danny on his side or out of the way, he could try to use Kim as a pawn. Maybe he tries to manipulate Kim into thinking Danny is the bad guy, or he uses his ghost powers and wealth to create problems for both of them.
I think Gaara could be an ally. Gaara has control over sand and is very powerful. He and Naruto have a mutual respect for each other. Additionally, Jiraiya. Jiraiya was Naruto's mentor and has a vast knowledge of different jutsu and combat techniques that could be used against Omega Shenron.
Iruka sensei might also be involved. He has always been like a father figure to Naruto. He could give Naruto the confidence he needs and maybe even help him plan the location for the proposal, perhaps a place that has sentimental value for both Naruto and Hinata related to Naruto's early days at the academy.
Perhaps an ancient evil entity that was once sealed away but is now breaking free. It wants to destroy all the good in both of their worlds, and Feyre and Aelin are the only ones who can stop it. The entity would be extremely powerful, with abilities that can counter both Feyre's and Aelin's powers to some extent.
Sasuke would be a likely survivor. His combat skills and intelligence would serve him well in a post - apocalyptic world. He could quickly assess threats and use his Sharingan to detect any hidden dangers, like approaching zombies or other hostile survivors.
It could be Ino. She's confident and outgoing, so she'd likely put on a great show. Her stylish nature would probably make her choose a trendy and popular song, and she'd perform it with lots of flair, which would impress the judges.
It could be an evil Sith Lord who discovers the existence of the Naruto universe and wants to conquer it. He sees Naruto and Ahsoka's alliance as a threat and tries to stop them. This Sith Lord might use his dark side powers to corrupt some of the characters from the Naruto universe and turn them against Naruto and Ahsoka.