Gray would be extremely angry. He would immediately start looking for clues as to where Natsu was taken. His ice - make magic would be on full display as he might freeze things in frustration while trying to figure out the situation.
I'm not sure if there is such a specific fanfiction. There are a vast number of fanfictions out there, and while some might explore unique and even out - of - the - ordinary concepts like this, it would take some searching through relevant fanfiction platforms to determine its existence.
One possible plot could be that the kidnappers are from a rival guild who want to use Natsu as leverage against Fairy Tail. Gray could then embark on a solo mission to rescue Natsu, facing various traps and enemies along the way. He might have to use his ice - make magic in creative ways to overcome obstacles.
Another interesting one is 'Gray and Natsu: Beyond the Guild'. This fanfic explores what would happen if Gray and Natsu left the Fairy Tail guild for a while. They end up in a strange land where they have to rely on each other to survive. It delves deep into their friendship and shows how they grow as individuals and as a team. The descriptions of the new land are really vivid and add to the overall charm of the story.
Romance is also a big theme. Many fanfictions play around with the idea of a romantic relationship between female Natsu and Gray. They might have some cute moments like blushing or secretly caring for each other.
One common theme is the rivalry - turned - into - attraction. Since Natsu and Gray are rivals in the original Fairy Tail series, in the lemon fanfics, this rivalry can often be a starting point for their relationship to develop into something more passionate.
One possible story could be an adventure where Gray, Lucy and Natsu team up against a powerful dark wizard. Gray could use his ice - making magic, Lucy her celestial spirits, and Natsu his fire - dragon slayer magic to overcome various obstacles and defeat the enemy.
Well, 'Gray and Natsu: A Different Kind of Bond' is a great one. It shows their relationship evolving in a new light, with some really engaging plot twists. Then there's 'Fusion of Gray and Natsu' which combines their powers and personalities in an exciting way within the fanfic. Also, 'Gray and Natsu: Unseen Emotions' is quite good as it focuses on the emotions that are usually not shown in the original series but are explored in the fanfic.
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.
Lucy might also be an ally. She has her celestial spirits that could be a great help. For example, some of the spirits like Leo could be very powerful in combat against the dragons. Also, Erza would be a great addition. With her various armors and combat skills, she could hold off a large number of dragons while Natsu focuses on the more powerful ones among the 10000.