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.
Some of the more independent lords or ladies in Westeros could be allies. For instance, Lady Brienne of Tarth. She is a paragon of honor and if the bastard displays honorable intentions, she could offer her sword and protection. Also, the wildlings might see the bastard as a way to gain some influence in the south. If the bastard offers them something in return, like a place to settle or resources, they could form an alliance.
One great male OC bastard fanfiction is 'The Bastard of Winterfell's Rise'. It follows an original character who is a bastard in the North. The story is engaging as it shows his struggle to gain acceptance in a society that looks down on bastards. He uses his wits and combat skills to make a name for himself.
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.
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.
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.
One idea could be that Cersei's bastard rises to power in secret. He or she could form alliances with unlikely characters, like some of the smallfolk who have been overlooked. They might use their wits and charm to gain support, all while hiding their true identity from the powerful families in Westeros.