I have thought about this theory for a while although I don't have concrete proof about its correctness. Would it be possible that Dr. Vegapunk is Usopp's grandfather? Because: The word 'Vega' from Vegapunk's surname can refer to a star in the constellation Lyra and most of Usopp's attacks are star themed (Green Star, Fire Bird Star, Gunpowder Star, etc.). This may be Oda's way to hint about their relationship. Both of them has skills to invent weapons. Vegapunk made Pacifistas and Usopp made Nami's Clima-Tact. Both of them has knowledge about physics and know how to make use of it. Vegapunk is well-known from his inventions and Usopp kind of applied rocket science to shoot Bag Worm during the Dressrosa Arc. My theory is that Usopp doesn't speak about his relationship with Vegapunk because he doesn't know that Vegapunk would be his grandfather because Yasopp never told Usopp about it. As a child Yasopp lived normal life with his parents before Vegapunk's illegal researches were revealed to his family and he was arrested by the World Government. After this Yasopp and his mother moved to another island. Later Vegapunk was employed as Marine scientist but Yasopp and Vegapunk didn't see each other anymore because Marines were watching Vegapunk and Yasopp hadn't any way to contact with his father. When Vegapunk started to get reputation as a Marine scientist and his son started a family, Yasopp decided not to tell his wife or son about his relationship with Vegapunk because he was afraid that they will be targeted by the World Govenrment, pirates or some vengeful people who had been in trouble thanks to Vegapunk's work. Vegapunk in turn never heard that he has a grandson because of the lack of communication between father and son. So, what do you guys think about this?