In the Fairy Tail universe, Igneel is extremely powerful with his fire - based magic. However, Acnologia is known as the most powerful dragon. I think Acnologia would win. His power is on a different level and he has shown to be nearly invincible in most battles.
One epic moment could be when Igneel unleashes his most powerful fire attack, a huge blaze that engulfs the area, and Acnologia just withstands it with his dark aura. Another could be when Acnologia lunges at Igneel with his massive jaws, and Igneel dodges at the last second.
In many Fairy Tail fanfictions, the battle between Natsu and Acnologia is often depicted as extremely epic. Natsu's fiery determination and his unique dragon - slayer magic make for an interesting contrast against Acnologia's overwhelming power. Some fanfictions might have Natsu discover new forms or powers in order to stand a chance against the fearsome Acnologia.
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.
Some writers characterize Acnologia as a misunderstood loner. They show that his actions of destruction are a result of his past traumas. Others might make him more of a power - hungry villain who just wants to rule over all. There are also those who give him a sense of humor, which is a very different take from his usual brooding and menacing self in the original series.
Well, adapting to Acnologia's body could be a complex process. They would have to learn how to control the raw power within it. This power might be overwhelming at first, and they could accidentally cause destruction. To adapt, they might start by testing their limits in isolated areas. Emotionally, they would also have to come to terms with the fact that they are now in the body of a feared being. They might try to use their memories of Fairy Tail to bring out the good in this new form, like using Acnologia's strength to protect the weak as they did in their previous life.
In some fanfictions, Acnologia is made out to be Natsu's father. But this is not part of the original canon of the Fairy Tail series. Fanfiction writers often like to create such unexpected family relationships to add new twists to the story.
In a 'naruto is igneel fanfiction', perhaps Naruto is transported to a world where Igneel exists. There, Igneel sees Naruto's potential and decides to train him. Naruto has to adapt to this new world and its rules while learning from Igneel. He could gain dragon - like powers, which he then combines with his ninja skills to face new and more powerful enemies.
It could mean that in a Naruto fanfiction, the character Acnologia from another series has been incorporated into the Naruto universe. Maybe the writer wanted to create an interesting crossover by bringing in this character's unique traits or powers into the Naruto - inspired story.
One popular female Acnologia fanfiction might be those that explore her backstory. For example, there could be a fanfic that delves into her early days as a dragon slayer and how she became the powerful being she is. It might show her training, her relationships with other dragon slayers at that time, and the events that led to her transformation into a more menacing figure.