A unique idea could be Smaug being tamed by Bran using his warging abilities. Bran, with his connection to animals and his growing powers, might be able to reach out to Smaug's mind and form a bond. This would completely change the role Smaug plays in the 'Game of Thrones' universe.
One really out - there idea is that Smaug and the White Walkers have a connection. Maybe Smaug was originally from the far North and was corrupted by the same magic that created the White Walkers. So he comes to Westeros not just for gold but also to fulfill some dark purpose related to the White Walkers.
One interesting SMAUG 'Game of Thrones' fanfiction could be one where Smaug somehow ends up in Westeros. He might be drawn by the rumors of the vast treasures held by the various lords. Another could be a story where he forms an alliance with one of the powerful houses, like the Lannisters, who are known for their love of gold.
In a 'Game of Thrones' and 'Sherlock' crossover, Sherlock's deductive skills could be used to solve the mysteries in Westeros. He could figure out who really poisoned Joffrey or find out the secrets of the Faceless Men. It would be interesting to see how his logical mind would interact with the more superstitious and political world of 'Game of Thrones'.
Smaug could potentially be a part of the magical elements in the 'Game of Thrones' fanfiction. He could be connected to the old magic that once existed in Westeros. Maybe his presence awakens other dormant magical forces. His actions could also influence the political landscape. For example, if he attacks a certain region, the lords there would have to divert their resources from the power struggles to deal with the dragon threat, which in turn could give an advantage to other houses.
Another concept is Thanos being a dragon rider. He could bond with one of Daenerys' dragons and use it to his advantage. He might use the dragon's power to conquer kingdoms and also use his own powers and the Infinity Stones to enhance the dragon's abilities, creating a truly unstoppable force in the Game of Thrones world.
One idea could be that the dragons in Game of Thrones are attracted to the magic of the Genie in Aladdin. So, Daenerys has to travel to Aladdin's world to find out why. Another is that Joffrey is transported to Agrabah and has to learn to survive without his royal status. He might end up working in the market, much to his dismay.
Smaug could be a terrifying and powerful antagonist. He could be lured to Westeros by the smell of gold and riches in the various kingdoms. His ability to fly and breathe fire would make him a formidable foe against any army or dragonriders in the Game of Thrones universe.
There might be a fan fiction where Smaug forms an alliance with a powerful house in 'Game of Thrones'. For example, he teams up with House Lannister. The Lannisters could use Smaug's power to defeat their enemies, while Smaug gets a share of the wealth and territory. This would create a whole new dynamic in the power struggle of Westeros.
One interesting 'Game of Thrones Smaug fanfiction' could be one where Smaug somehow finds his way to Westeros. Maybe he is drawn by the power struggles and the vast treasures held by the various houses. He could interact with characters like Tyrion, who is known for his intelligence and might see a way to use Smaug to his advantage in the game of thrones.
Another plot could involve the Prince of Darkness being a key figure in a prophecy that the maesters have long ignored. As he emerges, he begins to gather followers who believe he is the answer to the chaos in Westeros. His actions could lead to a split between those who follow the traditional religions of the land and those who are drawn to his dark allure.
Smaug would probably see most of the Game of Thrones characters as either insignificant or as potential thieves of his treasure. He might interact with the more powerful ones, like Daenerys, with hostility as she also has dragons. He would likely view her as a rival for dominance in the skies.