…He is neither Jaguar D. Saul nor Gol D. Roger. You know why? There is a crap-load of evidence:
Luffy was born too soon. Roger was executed 23 years ago, and if Luffy is 17, well...do the math. Roger isn't exactly middle-aged either.
Roger is Luffy's hero. How is this proof? Well, the only reason he admires him is because of his infamy and having plundered the One Piece (whatever that is). The mayor of his village said "His dream...or his destiny", which can be debated somewhere else. Also, he is scared of his grandfather and wouldn't idolize the man he fears.
Jaguar encourages Nico Robin to find the ponelygraphs. Why on Earth would Luffy's grandfather do that? Luffy is determined to find the One Piece, and Nico Robin wants to find the pnelygraphs (which undoubtedly are connected to One Piece). His grandfather must be a man who views justice the same way as the world govt. and he must have told Luffy not to pursue this dream. Otherwise...LUFFY WOULDN'T BE SCARED.
This isn't some ground-breaking, revolutionary theory/conclusion, but I thought I should clear it up since so many people are fond of the idea.