One great 'Game of Thrones Robert Arryn Fanfic' is 'The Rise of Robert Arryn' which shows his growth and development in a more positive light. It gives him some agency and intelligence that he didn't really get in the show.
One possible fanfiction could start with Robert Baratheon as a young man. He is full of ambition and dreams of a united Westeros under his rule. He trains hard in combat, his sword becoming an extension of himself. He meets his future friends, like Ned Stark, during these formative years. They share tales around the campfire and make grand plans for the future. As he grows older, he participates in battles, his victories making him more and more renowned. His charisma draws people to him, and soon he is seen as a natural leader. But with power also comes temptation, and he has to face the challenges of politics and intrigue that come his way.
Maybe in the fanfiction, his excessive pride got in the way. He was so confident in his military might from his past victories that he underestimated the threats within his own kingdom. This led him to be complacent, and when the challenges arose, he was ill - equipped to handle them.
Many 'Game of Thrones Robert Arryn Fanfic' are unique because they often blend different elements of the existing lore. Writers might combine the political intrigue of the Vale with Robert's personal growth. They can also introduce new characters that interact with Robert in ways that add new layers to the story. Some fanfics might even change the setting, like sending Robert on a journey outside the Vale, which creates a whole new set of adventures and challenges for him.
One interesting fanfiction could be one where Robert Baratheon survives and has to face the challenges of ruling Westeros in a more peaceful way. He might have to deal with political intrigues without the use of brute force as much. Another could be a story about his youth, exploring his relationship with Ned Stark before all the chaos of the Iron Throne began. And there could also be a fanfic that focuses on his relationship with his children, had he been more involved in their upbringing.
Robert was a great warrior in 'Game of Thrones'. His war stories are full of battles and glory. One of the most famous was his fight against the Targaryens. He led the rebellion that overthrew them. His warhammer was his signature weapon in these battles.
One possible alternate storyline could be that Robert manages to uncover the truth about Cersei's infidelity earlier. This would lead to a major political upheaval in King's Landing. He might exile Cersei and her children, which would change the power dynamics completely. With no Lannister influence at court, Ned Stark might have more sway, and the events leading to the War of the Five Kings could be averted.
Some of the secondary characters could include those who interact with the bastards. It could be a mentor figure, an old knight who takes one of the bastards under his wing and teaches him the ways of war and honor. Or it could be a love interest, a noble lady who falls for one of the bastards despite the social barriers, adding a romantic sub - plot to the story.