The devils that oppose Rias' group could be the antagonists. They might see Naruto as an unknown threat and try to eliminate him. Also, there could be some dark magic users who don't want Naruto interfering in their plans in Rias' world.
Maybe there are other powerful beings from different dimensions who sense Naruto's arrival in Rias' world and view it as an opportunity to cause chaos. They could be the main antagonists. These beings might be jealous of the potential power that Naruto and Rias could gain if they combine their abilities, so they try to stop them from working together.
There could be some traditional antagonists from Naruto's world that somehow follow him into Rias' world. For example, rogue ninjas or some of the old enemies of the Hidden Leaf Village. They could team up with the local villains in Rias' world to oppose Naruto and Rias. This would create a complex web of enmity as Naruto has to deal with both old and new foes.
One potential antagonist could be a rogue ninja who also has aerokinesis but uses it for evil. He might be jealous of Naruto's natural talent and try to undermine him at every turn. Another could be a powerful sorcerer from a far - off land who has come to the Naruto universe specifically to find someone with aerokinesis to either steal their power or use them as a tool. There could also be a group of anti - ninja who see Naruto's new power as a threat to their plans of overthrowing the ninja system and they try to eliminate him.
Well, Naruto being the son of Rias could have some interesting allies. Firstly, Rias herself would be a great ally, teaching him about the devil society and how to use his powers. Then, there could be other devil clans that are friendly to Rias' clan and would support Naruto. In the ninja world, his old sensei or comrades who respect his decision to embrace his new identity could be his allies. Also, there might be some mythical creatures in the devil realm that recognize Naruto's pure heart and decide to fight alongside him.
It could be the traditional enemies from Naruto's world, like the Akatsuki. They might see the union of Naruto and Rias and their unborn child as a threat to their plans for world domination. They could try to kidnap Rias to use her as a bargaining chip against Naruto or to study the unique nature of their child.
Perhaps Orochimaru could still be an antagonist. In this fanfiction, he might be jealous of Naruto's new god - like gaming powers and try to steal them for his own experiments.
There could be a jealous fellow ninja from within the village who feels that Naruto is getting too much attention for his blade - mastery. This ninja could turn against Naruto and become an antagonist, perhaps even teaming up with external forces. Also, an ancient evil entity that was sealed away long ago might be awakened by the power of Naruto's blade and seek to destroy him and the village to regain its freedom.
There could be new antagonists created specifically for this fanfiction. Maybe some other ninja from a far - off land who are jealous of Naruto's power. They could come to the Hidden Leaf Village to challenge Naruto and steal his speed - related secrets.
One potential antagonist could be Orochimaru. He might be interested in Naruto's Mokuton powers for his own evil experiments. He could try to capture Naruto to study how he awakened the Mokuton and use that knowledge for his own gain.
The villagers themselves could be antagonists in a way. Their fear of Naruto's 'demon' side might lead them to try and ostracize or even harm him.
A group of rogue ninjas who have made the island their base could be antagonists. They might see Naruto as a threat to their operations on the island.
One potential antagonist could be Orochimaru. He is always after power and the power of the Nibi within Naruto would be very appealing to him. He might try to capture Naruto to extract the power or use him for his own evil experiments.