In this kind of fanfic, the villains' roles would be very diverse. Bane, for instance, might see the zombies as a new challenge to overcome and prove his strength. He could either fight them head - on or try to control them. Catwoman, being a more morally ambiguous character, could go either way. She might steal supplies from survivors or she could help Batman if she thought it was in her best interest. The villains' actions would add a lot of complexity and twists to the story of a Batman zombie apocalypse.
The villains could either be a bigger threat or an unexpected ally. Some villains might see the zombie apocalypse as an opportunity to take over Gotham without Batman interfering. But others, like Poison Ivy, might actually try to find a way to stop the zombies as they don't want their 'playground' to be destroyed.
Well, Batman is a master of stealth. In a zombie apocalypse in this fanfic, he could easily move around undetected by the zombies. He has a wide array of gadgets that could be useful. His grappling hook, for example, could help him reach safe places above the ground where zombies can't reach. Also, his batarangs could be used as both a weapon to take out zombies from a distance and as a distraction.
Toga might be interested in the zombies in a different way. Since she has a fascination with blood, she could study the zombies to see how their blood or 'undead' state works. However, other villains might just be in it for themselves. Dabi, for example, might just burn his way through the zombie hordes without much thought for a grand plan. He'd be more focused on his own survival and causing chaos.
A possible plot could be that an unknown enemy uses a powerful jutsu that accidentally creates zombies. The story could start with Naruto and his friends in Konoha noticing strange behavior among some villagers. As the situation worsens, they realize it's a zombie outbreak. They then have to band together, find out the source of the outbreak, and try to stop it while also protecting the remaining survivors.
Sonic's speed allows him to be a great scout. He can cover large areas in no time to find safe places or look for resources. His ability to spin dash can also be used as an offensive move against zombies. Tails, on the other hand, could use his two tails to fly and get a better view of the surroundings. He could also use his knowledge of mechanics to jury - rig weapons from the debris around. For instance, he might make a makeshift flamethrower using some old parts and a fuel source, which would be very effective against zombies.
The ninja in the Naruto world are used to combat and danger. In a zombie apocalypse, characters like Kakashi could use his intelligence and wide range of jutsu. He might use his Earth - Style: Earth Wall to create barriers against the zombies. As for Hinata, her Byakugan could be used to see zombies from afar, giving the group early warning. And the teamwork among them, which is a staple in the Naruto series, would be their greatest strength in this new and terrifying situation.
The members of Team JNPR could also be likely survivors. Jaune has grown a lot in his skills. His leadership and swordsmanship could be useful. Pyrrha, if she were still alive in this fanfic scenario, would be a great asset with her combat abilities. Ren's calm and collected nature, along with his stealth and combat skills, would help him survive. And Nora, as mentioned before, would be a great addition to the group of survivors. They could work together with Team RWBY to face the zombie apocalypse.
The Joker's gas can be a major source of conflict. It can put the Batfamily in immediate danger, forcing them to find antidotes or ways to avoid it. For example, the gas could make them hallucinate, seeing their worst nightmares come to life.
Well, the characters in Fairy Tail are used to facing tough situations. In a zombie apocalypse, they would probably stick together as a guild. Wendy could heal any of her comrades who got injured during battles with zombies. Lucy might use her Celestial Spirits, like Taurus with his great strength, to physically fight the zombies. And Happy could be a lookout, using his flying ability to spot approaching zombie hordes from above.
Sonic might adapt by using his environment in new ways. For example, he could run up walls and jump off them to avoid zombies. Knuckles could start using his knowledge of the land to find safe places. Since he knows a lot about the different zones in the Sonic world, he could lead Sonic to places that are less likely to be swarming with zombies. And both of them would have to learn to work together more closely than ever, as the threat of the zombies is much different from their usual foes.
Harry Potter himself would be a likely hero. He has already faced Voldemort and numerous other dark threats, so he has the courage and experience. His ability to think on his feet and his connection to the powerful Patronus charm could be very useful against the zombies.