It could be an old enemy of Ryan's who has returned stronger than ever. This antagonist might have a vendetta against Ryan specifically and sees the addition of the OC as just another obstacle in his path to revenge.
The main antagonist could be a corrupted former Power Ranger. This ex - ranger was once a friend of Ryan's but has been turned to the dark side. He now wants to destroy the Lightspeed Rescue team and uses his knowledge of their tactics against them. The OC, being new, doesn't fall for his old tricks and is able to come up with new strategies to combat him.
Well, it could be about the new character (OC) joining the Power Rangers Lightspeed Rescue team and having some sort of adventure with Ryan. Maybe they face a new and powerful enemy together and the OC has some unique skills or abilities that help the team in a way they hadn't expected.
Definitely Ryan Mitchell. His complex backstory as the Titanium Ranger gives a lot of material for fanfiction. He has the whole arc of being a former villain and then turning into a hero, which can be explored further. Writers can delve into his internal struggles, his relationship with his sister Dana, and how he continues to prove himself as a true Ranger after his dark past. Joel Rawlings, the Blue Lightspeed Ranger, is also popular. His scientific knowledge and quick thinking can be the basis for stories where the Rangers have to use their brains as well as their brawn to solve problems.
Maybe the villains from Power Rangers Lightspeed Rescue find a way to enter the Naruto universe. The Rangers follow them and team up with Naruto and his friends. They could teach each other their fighting techniques. The Rangers could show Naruto how to use their high - tech weapons, while Naruto could teach them about chakra and ninjutsu.
A strict teacher at Rydell High could be the main antagonist. The teacher might disapprove of the OC's influence on Michael, thinking that the OC is distracting Michael from his studies. The teacher could constantly be giving them detention or trying to keep them apart, which would create conflict in the fanfic.
A new, original villain created just for this fanfic could be the main antagonist. This villain could be a powerful sorcerer who has been observing the Avengers from the shadows. When the OC is reborn, they sense a disruption in the magical balance of the world. Believing the OC to be a threat to their own plans of ruling the world through magic, they set out to eliminate the OC and in the process, come into conflict with the Avengers.
It could be a powerful Arrancar. In the Bleach universe, Arrancars are often antagonists. A particularly strong and cunning one might see the oc as a threat to their plans. Maybe the oc's presence disrupts the Arrancar's scheme to take over a certain part of the Soul Society or gain more power.
A powerful and greedy poacher could be a great antagonist. He's only interested in capturing the Ninetaels for its value and doesn't care about the Pokemon's well - being or the balance of nature.
It could be a new character. Maybe a powerful ninja from a far - off land who has heard rumors of Naruto's strength and is determined to draw it out and defeat him to prove his own superiority. This antagonist might be secretly spying on Naruto and trying to find ways to force him to show his true power.
Maybe the main antagonist is an agent of the World Government who is secretly operating in the West Blue. They view Shinshi's activities, whether they are innocent or not, as a potential disruption to their plans in the area. So they try to stop Shinshi by any means necessary, setting up various traps and sending assassins after him.
Perhaps an existing 'One Piece' character could be turned into the main antagonist in the context of this fanfic. For instance, a marine officer who sees the OC as a threat to the world government's order. Since Viola has ties to Dressrosa, which has its own complex relationship with the world government, this marine could target them both. The officer might use underhanded tactics to try and stop the OC and Viola from achieving their goals.