Gray is another character. His ice - magic can complement Natsu's fire - magic. They might team up to fight enemies or overcome obstacles in the way of saving Fairy Tail. Sometimes they bicker while doing so, which adds some humor to the story.
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.
Typically, Natsu and Lucy are often the main characters in Fairy Tail related fanfictions, and they could very well be the ones involved in a 'gale kiss' scenario. Their dynamic relationship, with Natsu's brashness and Lucy's more level - headed nature, makes for interesting stories when a special moment like a kiss occurs.
Mirajane is a popular choice too. She has a mysterious side to her, and in the fanfictions, she could use her unique abilities to help Natsu or be involved in the events that the Fairy Tail members are watching. Also, Laxus might be included. His powerful magic and complex character can add depth to the fanfiction stories, especially when his relationship with Natsu and the rest of the guild is explored.
Most likely, the other members of Fairy Tail would be involved. For example, Lucy might be there. She could be in distress during the attack that Natsu is trying to fend off, or she could be one of the first to find Natsu after he has fallen. Gray could also be involved, perhaps he was fighting alongside Natsu or arrives shortly after to see what has happened. Erza too, as a strong and important member of the guild, she may play a role in the events following Natsu's fall.
Definitely Lucy. Their bond is so strong that if there were any cracks in it, like if Lucy started distancing herself from Natsu without a clear reason, it would be devastating for him. Gray is also a candidate. They've been through so much together, and if Gray were to say or do something really hurtful, like blaming Natsu for a big mistake that wasn't his fault, it could lead to heartbreak. Juvia might be involved too. If she tried to keep Lucy away from Natsu for her own love for Gray, it could cause problems for Natsu and lead to his heartbreak.
Well, in many of such fanfictions, Natsu might use his unique fire - based magic in really creative ways to save the Fairy Tail guild. For example, he could combine his dragon slayer magic with some unexpected elements from the environment to defeat a powerful enemy threatening the guild.
One common way is by using his combat skills. Natsu is a strong fighter, so he often defeats the villains who are threatening Lisanna.
Obviously, Hermione is the central character. Harry and Ron are often also important. They might be in trouble and Hermione has to save them. For example, they could be captured by Death Eaters and Hermione devises a clever plan to rescue them.
Seilah is also a main character in this context. She has her own set of powers and a rather complex personality in the Fairy Tail universe, which can be further explored in fanfiction.
Lucy could be a main character. She was always close to Natsu in the original Fairy Tail. So when Natsu returns as an 'ex' and meets the guild again, she would likely play a big role in how he is received and in the development of the story.