One interesting plot could be that the Fairy Tail members hold a pool party to celebrate a big victory. During the party, there are various fun competitions like the best pool trick competition. Gray might accidentally freeze part of the pool, causing chaos but also lots of laughter.
Happy, Natsu's sidekick, is almost always present in these fanfictions. His ability to fly can add an extra element of fun to the pool party, like dropping water balloons from above. And of course, the guild master Makarov might be involved, either as the organizer or as an observer, adding his wisdom and humor to the whole pool party scenario.
Well, there could be a lot of things. Maybe Lucy would show off her new swimsuit and Gray would make ice sculptures on the beach. The Fairy Tail guild members could also have a big barbecue on the beach, with Erza trying to keep everyone in line while they enjoy the food and the ocean view. They might even discover some mysterious sea creature that leads to an unexpected adventure.
You could have a 'Magic Swap' party. In this party, the Fairy Tail members temporarily swap their magic abilities with each other. This would lead to some really funny and interesting situations. Natsu trying to use Gray's ice magic and constantly slipping on his own ice creations, or Erza using Lucy's celestial magic and trying to summon spirits in a very Erza - like strict way.
Natsu would probably be the most entertaining. His fiery and impulsive nature would lead to all sorts of comical situations at the beach. He might try to swim in lava - like hot water or accidentally burn someone's beach towel while trying to dry himself off.
They could interact more casually. In the original series, there are often battles and serious missions. But at a party, they might just joke around and share funny stories. For example, Gajeel and Natsu could have a friendly competition about who can eat the most food at the party instead of fighting over something serious.
A Yokai Watch Pool Party Fanfiction could start with the invitation to the pool party being sent out among the yokai community. Once at the party, the yokai could be divided into teams for a relay race in the pool. Some yokai might be shy at first and just sit on the edge, but as the party progresses, they get drawn into the fun. There could be music playing, and the yokai are dancing around the pool. Also, there could be a mystery or a small problem that occurs during the party, like a missing pool toy, and the yokai have to work together to solve it.
Well, one appropriate pool party story could be about a pool party where there was a really fun water volleyball tournament. Everyone was divided into teams and there was lots of laughter and friendly competition. The best part was when a little kid joined in and managed to score a point for his team, making everyone cheer.
It adds more content. The official Fairy Tail series has a limited number of episodes and chapters. Fanfiction fills in the gaps. For example, it can create stories about what the characters were doing during the time - jumps in the original story.
If Lucy never joined Fairy Tail, the friendships and relationships among the characters would form in a very different way. Also, many of the story arcs might not unfold as they do in the original plot.