A really interesting plot could be that a powerful dark wizard has cursed the highschool in the 'Highschool of the Dead' world, turning it into a battleground. The Fairy Tail guild is called upon to help. They discover that the curse is tied to an ancient artifact hidden within the school. As they search for the artifact, they face not only the zombies but also traps and illusions set by the dark wizard.
One possible plot could be the characters from Fairy Tail having to survive in a highschool during a zombie apocalypse. They could use their magic powers in unique ways to fight off the zombies and protect their classmates. For example, Natsu could use his fire magic to burn groups of zombies at once.
Well, a cool plot might involve the merging of the Fairy Tail world with the 'Highschool of the Dead' setting. The students in the highschool start developing magical abilities similar to those in Fairy Tail. However, there are also some dark forces at work that are trying to take over the school. The Fairy Tail characters have to train the students to use their new powers to defend the school.
Maybe there could be a big competition at the highschool, like a magical sports event. Different characters from Fairy Tail would form teams and compete, with rivalries and friendships being tested. For example, Natsu's team against Gray's team, and there could be some comical moments as well as intense battles.
One possible plot could be that the zombie apocalypse hits the ninja high school. Naruto and his friends have to use their ninja skills not only to fight the zombies but also to protect the other students. They could form teams and set up barricades around the school, using their jutsu for both offense and defense.
One possible plot could be that the guild goes into deep mourning. Gray might blame himself, thinking he could have protected Natsu. Lucy could be in a state of shock and disbelief, and she might start writing in her diary about all their adventures, as if reliving the memories to deal with the pain.
I think Lucy would be really interesting. She could use her Celestial Spirit Magic in creative ways to deal with the situation in the highschool during the apocalypse. Maybe she could call upon spirits to help barricade areas or to scout for danger.
Maybe the dragons from Fairy Tail are related to the dragons in Highschool DxD in some ancient way. So, characters like Igneel might have a connection to the dragon in DxD. This could lead to a search for the truth about their origins and a fight against a common enemy that threatens both worlds.
One idea could be that the magic from Fairy Tail somehow leaks into the world of Highschool DxD. For example, Natsu's fire magic could be mistaken for a new type of devil power. Another could be a crossover of characters where Erza teams up with Rias to fight a common enemy. Maybe a powerful dark guild in Fairy Tail teams up with some rogue devils in Highschool DxD.
Well, perhaps the bet could be about a race. Bacchus bets that he can run faster than a particular Fairy Tail mage. In the process of preparing for this race, the Fairy Tail mage discovers some new abilities within themselves. And as they compete, it's not just about winning the bet but also about the respect and honor of their respective guilds. This could also show how different the cultures of their guilds are when it comes to competition.
Well, perhaps there could be a situation where an evil force in the Fairy Tail universe is creating portals to other dimensions to gather powerful allies. Deadpool accidentally stumbles through one of these portals. The Fairy Tail guild has to figure out how to deal with this strange, talkative, and unpredictable new 'ally'. Deadpool, on the other hand, is just excited to be in a new world full of magic and new things to fight. He could end up teaching some of the Fairy Tail members his unique combat moves, while also learning some magic from them in return.
A plot could involve a magic competition in the Fairy Tail guild. Mirajane and Freed both enter. Mirajane wants to show off her new transformation magic, and Freed aims to prove the power of his rune magic. As they compete, they start to learn from each other. Mirajane might pick up on Freed's precise and calculated approach to magic, while Freed could be inspired by Mirajane's bold and fearless use of her powers. This could lead to a friendly rivalry that also strengthens their bond within the guild.
A cool plot might be that a powerful dark wizard in the Fairy Tail world discovers a way to control some of Ben's aliens. Ben has to seek help from Fairy Tail mages to break the control and then they join forces. The Fairy Tail members are amazed by Ben's ability to transform and his alien powers, and they work together to stop the dark wizard's plan to take over both worlds.