Before One Piece became the pirate series we know and love today, Eiichiro Oda created a precursor called Romance Dawn. Luffy appears in these stories along with Shanks.
One Piece Romance Dawn is being adapted into a PSP RPG. A scan from V-Jump shows Shanks patting a young Luffy on his head. One Piece Romance Dawn: Adventure at Daybreak uses a command battle system. A screenshot shows a solo Luffy about to pound four sailors with his fists. The game has a 3D world to explore and players will be able to customize Luffy.
One Piece Romance Dawn: Adventure at Daybreak is slated for release this winter in Japan.