A simple way would be for a Fairy Tail wizard who can control fire to team up with a vampire who has super speed. The vampire can quickly move the wizard around during a battle, and the wizard can use their fire magic to attack enemies.
Naruto has his Nine - Tails chakra which is extremely powerful. If Lucy were to use her magic to somehow channel that chakra into a new form of attack, it could be very effective. For example, she could create a magic circle that concentrates the Nine - Tails chakra and then release it as a beam of energy. Or perhaps Naruto could use his chakra to enhance the abilities of Lucy's spirits. He could transfer some of his chakra to a spirit she has summoned, making it faster, stronger, and more durable, while the spirit could use its own unique abilities to help Naruto in combat, like a flying spirit could give Naruto aerial support during a fight.
In some fanfictions, Natsu's dragon slayer magic and Mavis's Fairy Law could be combined in a unique way. For example, Natsu could use his fire to enhance the power of Mavis's Fairy Law, making it more destructive against their enemies.
Kagome has her spiritual powers like purifying arrows. In the Vampire Knight world, vampires have their own abilities such as enhanced strength and speed. They could combine powers by Kagome using her purifying arrows to weaken enemies that are resistant to the vampires' physical attacks. For example, if there's a powerful dark creature, the vampires could hold it in place while Kagome fires her arrows to purify it.
Well, often in such fanfictions, the Rinnegan is used as a sort of bridge between the Naruto and Fairy Tail universes. It might give a character the ability to travel between the two worlds. And when it comes to combat, the Rinnegan user could analyze the magic in Fairy Tail like they analyze chakra in Naruto. They could then use this knowledge to counter magic attacks more effectively. Also, the power of the Rinnegan could be enhanced by Fairy Tail's magic, making the character even more powerful than in the original Naruto series.
For characters like Wendy, if she got a Devil Fruit power, she'd need to be careful not to overexert herself. Since she's a healer, she might get a Paramecia - type Devil Fruit that gives her the ability to heal more effectively. But she'd have to learn how to balance it with her Sky Dragon Slayer Magic. And characters like Elfman would need to find a way to use their Devil Fruit power in harmony with their Take - Over Magic. If he got a Zoan - type Devil Fruit, he'd need to figure out how to transform between his normal form, his Take - Over forms, and his Devil Fruit form without getting confused or losing control.
Well, Mario's jumping and acrobatic moves in Smash Bros could be paired with Happy's flying ability from Fairy Tail. Happy could carry Mario to new heights in the battle area, allowing Mario to perform his aerial attacks from greater heights. And Mario's fireball ability could be enhanced with some Fairy Tail fire - based magic for extra damage.
The magic in Fairy Tail could be combined with Pokemon elements by having spells that are related to Pokemon types. For instance, a water - mage could use a spell to create a Pokemon - like water creature. Or a lightning - mage could summon an electric - type Pokemon - inspired bolt of energy.
In a high school setting, Jellal's personality can develop as he tries to fit in. He might be initially reserved and not very talkative, but as he gets involved in school clubs or teams, he starts to show his leadership skills and his loyalty to his friends. Erza's personality can be developed by having her face challenges outside of her comfort zone. For example, if she has to participate in a group project where she's not in control, she can learn to be more collaborative and less overbearing. Also, her relationship with Jellal can bring out a softer, more vulnerable side of her that we don't usually see in the original Fairy Tail.
One plot idea could be a new student, who is a vampire, arrives at Fairy Tail High School. They initially try to hide their true nature but end up getting involved in a school mystery that only their vampire abilities can help solve. Another could be a love story between a Fairy Tail student and a vampire student from another school during a joint event.
Perhaps there is a powerful artifact in the Fairy Tail world that has the ability to bestow Bleach - like powers. It could be an ancient relic that was created long ago by a powerful mage who had some connection to the Bleach universe. When the characters discover this artifact, they are each granted different Bleach powers based on their own personalities and magical abilities. For example, Erza might get a power related to her strong will and leadership skills.