just to throw in. Luffys Garp would be coby if we would want to force that idea...
Roger was younger than Garp and they were constant rivals, Coby is someone inspired by Luffy who has never rivalled him.
Luffy is the one standing up against "Rocks".
Luffy is the one standing up against the tenryuubito.
Luffy and Garp have similar personalities, with Luffy being even more extreme and outrageous than his grandfather.
Coby is a timid guy with a good heart who can muster true bravery in spite of fear.