An adventure could involve them traveling to different magical locations to solve a mystery. Gibbs would be out of his element but his sharp mind would still be useful. Harry could show him around Diagon Alley, introduce him to magical creatures, and they could work together to stop a plot that is endangering both the wizarding and Muggle worlds. They would face many challenges along the way, from dealing with unfriendly wizards to understanding complex magical spells and Muggle technology at the same time.
They could go on an adventure to find a missing magical artifact that has fallen into the Muggle world. Gibbs' skills in tracking and Harry's knowledge of magic could be combined to track down the artifact before it causes any harm.
He could go on adventures to discover hidden vaults within Gringotts. Maybe there are vaults that have been lost for centuries, filled with ancient and powerful magical artifacts. Harry could search for these vaults, facing various traps and puzzles along the way, similar to the challenges he faced in the Philosopher's Stone.
Harry could have adventures where he tries to help his parents' ghosts pass on to the afterlife properly. This would involve solving mysteries about their untimely deaths. Along the way, he'd meet other ghosts with their own stories, and they could join him on his quest. He might also find ancient spells that can interact with the ghostly world more fully during these adventures.
He could go on a journey to save Christmas. Maybe some dark magic has affected the North Pole and it's up to Harry to use his wizard skills and his newfound knowledge of the Christmas magic to fix things. He'd have to battle against dark elves or some sort of anti - Christmas spirits.
They could go on a quest to find a magical - technological hybrid artifact. The two of them would use their unique skills. Tony would use his tech to analyze the magical elements and Harry would use his magic to navigate through the enchanted areas they might encounter.
In a 'Harry Potter is Gibbs' nephew' fanfiction, the plot could start with Harry's discovery of his relation to Gibbs. This could lead to him spending time in the Muggle world, learning about Gibbs' work and life. As he grows, he might find ways to incorporate his magic into solving problems in the Muggle world that Gibbs and his team face. There could be a conflict where the wizarding world authorities are not happy about Harry's close ties to the Muggle world. At the same time, Gibbs' team might have a hard time accepting the existence of magic at first. But through various adventures and challenges, they all learn to work together. Harry could use his magic to enhance the forensics work in NCIS, like using a spell to detect lies during interrogations. And Gibbs could teach Harry about loyalty and perseverance in a way that is different from what he has learned in the wizarding world.
If they meet, there could be a lot of cultural clashes. Harry comes from a world where magic is normal and Gibbs is from a world where everything is based on scientific evidence and law enforcement. Harry might introduce Gibbs to things like butterbeer and thestrals. Gibbs could introduce Harry to things like NCIS procedures and how to handle a firearm. Their different perspectives could lead to some really interesting interactions and storylines.
In this fanfiction, Harry could have an adventure of uncovering the true power of Potter Manor. The manor may have been built on a powerful magical ley line, and as Harry explores, he discovers that the manor has the ability to amplify his magic. However, there are also dark forces that want to take over the manor for its power. Harry has to battle these forces while also learning how to harness the manor's power for good. He might also discover a hidden room with a prophecy about his future and the future of the wizarding world, which would set him on a whole new adventure to fulfill or change that prophecy.
Harry could get involved in a quest to protect a Spanish magical artifact from dark wizards. Since Spain has a rich history, there could be many such artifacts that are important to the Spanish wizarding community. He might have to travel across different regions of Spain, from the sunny beaches to the mountainous areas, facing various challenges along the way. He'd also encounter different types of Spanish witches and wizards, some friendly and some not so much, all while trying to keep the artifact safe.
One possible adventure is that they travel to a different dimension. The goblin friend has a special ability to open portals, and they end up in a dimension where magic works differently. There, they have to find their way back while also helping the local magical beings who are being oppressed by a powerful entity. Or they could go on an adventure to a hidden island where a powerful magical creature lives, and they need to get an ingredient from it to make a potion that can save Hogwarts from a curse.
He could have underwater adventures. The trident might give him the ability to communicate with sea creatures and explore the depths of the ocean. He could discover lost underwater cities or battle against sea monsters.