Smoker as the marine foil to Luffy. Technically this should be Coby, but I think Coby is far too inspired by Luffy and holds too much admiration for this to ever be taken seriously. Whereas Smoker could've served as a monolith for Luffy to butt his head against more in the story. We're constantly seeing Luffy's progress against a constant of an opponent until we get the satisfaction of Luffy matching and surpassing them.
He'd also serve as a good adversary in ideals. Luffy is all about freedom and Smoker isn't explicitly against that, they're both good natured and rebellious people on the opposite ends of the spectrum. And Coby, again, really doesn't fit this.
So I was very surprised when we get a lot of Smoker in Alabasta, a little after Skypeia in a singular chapter then nothing until Marineford. And Punk Hazard he was dethroned as any opponent for Luffy—relegated to bait for Law against Vergo. It's really disappointing because he was one of the more standout and cool characters that we followed early on.