Some are character traits, some are simply roles that need filling.
Luffy and Roger should be obvious. Roger was portrayed to be in a rivalry trio of sorts with two other pirates, Shiki and Whitebeard. At Sabaody, Luffy was portrayed as having a rivalry with Kidd and Law. Kidd fits Shiki more, as they are both rather hotheaded and outlandish. That leaves Law to being WB's resemblance, which makes some sort of sense as Law isn't as outlandish as Shiki, but rather is cool and levelheaded like Whitebeard with a OP DF like Whitebeard's. Law and Luffy have an alliance, but an unsteady one. Even so, while they are against each other in the end, they are on somewhat good terms. Roger and Whitebeard were the same in a way, though enemies, they got along more or less. Kidd and Luffy though have no such interaction, and have been purely portrayed as enemies so far, same as Roger and Shiki.
Smoker is compared to Garp because he's a Marine who is hunting Luffy. Luffy's parallel was Roger, who was hunted by the Marine Garp. Both Smoker and Garp go against the conventional Marine codes, often opting to act on their own, but still being allied to the Marines. I allude Coby to Sengoku because he is righteous, which Sengoku has exhibited in his ideology that the Marines are for the people, but not blood lusted like Akainu has been. I see Coby being the Fleet Admiral of his and Luffy's generation in the future.
