A soldier who fought against the aliens in the past could be the main character. They have the combat skills and experience, and now that the alien is resurrected, they feel it's their duty to protect humanity again. They might be haunted by the memories of the previous battles and be more determined than ever to win.
A young and curious journalist could be the main character. They stumble upon the story of the alien resurrection and are determined to expose it to the world. As they dig deeper, they get involved in the fight against the alien, using their wits and ability to gather information to help the resistance.
There could be a fearless scientist who discovers the alien resurrection and tries to stop it. Also, a tough military person who leads the fight against the resurrected aliens.
Since it's a resurrection story, there could be a character who has some past connection to the aliens. Maybe someone who was part of an earlier encounter and now has to face them again. And of course, there will probably be some expendable characters too, who are just there to add to the tension and horror when the aliens start attacking.
There could be a group of corrupted Quincy who view Ichigo's resurrection as an abomination. They believe that his power goes against the natural order of things. So, they band together to try and eliminate him. They might have some new and dangerous weapons or techniques specifically designed to counter Ichigo's resurrected powers. This group could be led by a very powerful Quincy who has a personal vendetta against Ichigo or the Soul Reapers in general.
All Might could also be a main character in a bnha alien fanfic. His status as a symbol of peace in the BNHA world could be extended to an intergalactic scale. He could be the one to bridge the gap between humans and aliens, using his experience and power to negotiate and fight when necessary. His weakened state could even be a plot point, as he has to rely on new alien technology or powers to regain his strength and continue to protect.
Vandal Savage is also a possibility. He has a long - standing enmity with the heroes. Maybe he has a new plan that involves using the resurrected heroes for his own evil purposes, or he's trying to stop their resurrection because he fears they'll thwart his future plans.
It could be a dark wizard or sorcerer who was the one responsible for the princess's death in the first place and doesn't want her to be resurrected. They might have their own dark motives like wanting to rule the princess's kingdom or gain some powerful magic from her death.
Perhaps a half - alien, half - vampire hybrid. This character was shunned by both species and blames Ben for making him feel even more like an outcast. He uses his unique powers to create chaos and tries to turn Ben into a vampire - alien like himself.
Obviously, Naruto would be one of the main characters. His never - give - up attitude and powerful chakra would be crucial in fighting the aliens. Sasuke would also be important, with his Sharingan and combat skills. Sakura, with her medical ninjutsu, could play a big role in treating the injured ninjas during the invasion. Kakashi could lead the strategic planning, using his vast experience.
There could be an alien race that has a very different ideology from the people of Remnant. For example, they believe in a survival - of - the - fittest concept at an extreme level. Their leader, who enforces this ideology and leads the invasion, would be the main antagonist. RWBY would have to oppose this leader's cruel ways and protect their own values.
One idea could be that the resurrected alien has some new powers. Maybe it can now blend into its surroundings like a chameleon, which adds a new level of danger. Another could be that the alien forms an alliance with a group of humans who are outcasts from society, and together they try to take over a powerful corporation.