I could see them joining the revolutionaries or bounty hunters, but even that's a bit of a stretch and only after some upheaval in the Marines.
Let's take Tashigi, who's been the target of conversation for some time for various reasons. If you remove her from her position as a Marine, there's really not much to her character. She's decent at swords (might be on Brook's level, but Zoro has her way out-classed, at least at the moment), smart, nice to people, has some comedy potential with Zoro and Sanji, knows at least one of the Six Powers, and …?? What makes her character even mildly interesting is her relationship with the Marines and how she goes about establishing justice in the world.
And then her superior, Smoker. Outside of his relationship with the Marines and the fact that he's after Luffy, he's not super interesting either, and I say this even as he's my favorite character! A logia, fights with a jutte, battle-smart, can instill loyalty, might have something going on with Hina, and ...?? Seems like most of his character is tied up in him being a rebel to the Marines. You take him out of the Marines, and then what? He needs some hierarchy to tell to eat shit after all. 
The reason people like or at least don't outright hate these characters is because as they are positioned in the story, as Marine opponents, they're interesting. Imagine if Tashigi hadn't been a Marine, and just some nice lady at Loguetown. I mean, there'd be even more Kuina speculation, but beyond that, she doesn't bring much to the table. Yeah, her swordsmanship and Kuina-look-alike whatever brought her into the story, but I don't think that's really enough to propel her through it and remain relevant. And I honestly can't picture Smoker as not a Marine initially. Without it or some structure, he just becomes a trash-talking tough guy, which we have plenty of.
I have a feeling X. Drake is our Marine-turned-pirate-because-of-corruption story (Kizaru basically says as much), so we wouldn't need Smoker and Tashigi to become pirates anyway to get that angle.
@loledinmypants:
According to enthusiasts Monet was argued as sensible and her story "still running" until Dofla is dealt with just recently. Perona back in the day had people supporting her until the chapter after the Sunny dove to go to FI (because maybe she snuck aboard!). What I'm saying is this "wait and see, I'll admit I'm wrong, but until then my crazy idea has merit" thing has been pulled before and drags out discussion of silly candidates longer than they deserve. Hopefully you can understand why to many you're sounding like the same thing with a different face to the fanboyism.
Apparently Oda is such an incompetent writer he can't pull off Smoker chasing Luffy down like Garp was said to have done with Roger without Smoker looking incompetent. And clearly the only solution to this is joining the crew. Because quitting your job to join the people your job is to hunt down is completely a show of competency.
And how would Smoker and Tashigi get angry at the WG after this arc? CC is tied to Dofla and iirc Smoker treats Shichibukai the same as any other pirate when said Shichi is being an evil bastard. If anything he'd hate pirates more because even the supposedly trustworthy ones are villainous fiends.
Even if Smoker and Tashigi quit the Marines because the WG is evil (before the inevitable final showdown/split in the Marines), why would they become pirates? They've been characterized as a moral force within the Marines that will do what they believe is "Justice" regardless of what the superiors say (Smoker has in particular). If anything they'd become vigilantes, bounty hunters, or join the Revolutionaries, the latter probably with some pirate hunting on the side for good measure, because Smoker'd probably tell Dragon to eat shit if questioned about pirate hunting (assuming Dragon'd even care).
I can understand that many want a logia and Smoker could fill that niche, but wtf would Tashigi do? Oh, I see. We definitely need Tashigi to join to stir up more Zoro drama and fuel the "women in OP" thread :ninja:. Oh wait, that'll just happen every time she and Zoro interact from now on.
Yes, please, put this on every page, this is a good post.
<3
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