Perhaps in the fanfiction, Igneel is not fully himself when he meets Fairy Tail. He has lost some of his memories due to a dark magic spell. The members of Fairy Tail have to work together to help him regain his memories while also protecting him from those who want to use his weakened state against them. They could go on a journey to find the source of the spell and break it.
One idea could be that Igneel appears in a time when Fairy Tail is facing an enemy they can't defeat. Igneel's power and wisdom help turn the tide. For example, he could teach Natsu some new and powerful dragon slaying magic that Natsu then uses to overpower the foe.
One idea could be that the Fairy Tail guild travels to the Naruto world somehow. They could team up with the ninja to fight a common enemy like a powerful dark magic user who has also invaded the ninja world. Maybe Natsu's fire magic could be compared and combined with Sasuke's fire jutsu in some unique ways during battles.
Well, in this fanfiction, Igneel's return could bring a huge shift in power dynamics. His relationship with Natsu might be explored more deeply, like they could have some intense training sessions. Also, it might lead to new quests for the Fairy Tail guild as Igneel's knowledge could be used to combat stronger enemies.
A group of rogue dragon slayers who have turned to the dark side could be the main antagonists. They envy Igneel's relationship with Natsu and Fairy Tail. They believe that dragons should rule over humans and plan to use their powers to subjugate the world. Igneel and Fairy Tail must stop them.
One idea could be that the next generation discovers a hidden spell from the old Fairy Tail days that has the power to either save or endanger the guild. Another could be a tournament where the newbies team up with old - school Fairy Tail members to compete against other guilds.
One possible plot could be that Igneel's arrival in Fairy Tail brings a new level of power and protection. He could start training the mages in new and powerful dragon - slaying magic, making Fairy Tail even more formidable against their enemies.
One idea could be a new character arriving at Fairy Hills and having to adapt to the unique living situation. They could face challenges like getting along with the different personalities of the Fairy Tail mages living there.
One significance could be the emotional boost for Natsu. After so long without Igneel, having him back would be a huge source of comfort and strength. Natsu could become even more determined in battles.
One idea could be Metalicana's backstory before he was known in the Fairy Tail world. Maybe exploring his early days as a dragon, his first encounters with other dragons or magical creatures. Another plot could involve Metalicana having a secret mission that is related to the future of the dragon slayers in Fairy Tail. It could also be interesting to have a plot where Metalicana has to team up with some of the Fairy Tail members in a time - travel adventure to save the past and thus ensure the future of the guild.
Another plot could involve a dark mage in the Fairy Tail universe summoning Deathwing to gain power. But the summoning goes wrong and Deathwing is only partially summoned. This weakened Deathwing starts causing destruction while also being hunted by Fairy Tail. The guild then has to decide whether to try and fully summon him to send him back or find another way to deal with him.
A cool plot could be that a powerful artifact in Skyrim is causing chaos and the Fairy Tail members are summoned to find and secure it. Along the way, they meet up with the Dragonborn. Some members might befriend the Dragonborn and help in their battles, while others might be a bit more skeptical and think the Dragonborn is a threat to their own power within the group.