Such Art Many Wow

The very first episode of the anime has an antagonist who's about twice as tall as vash (who is solidly in the 6 foot range) and improbably large characters continue to show up intermittently for the rest of the show. mind you, i can't think of any off the top of my head who don't have equivalent girth to match, but that puts rocinante solidly in the category of "rare species of twink" rather than "freak of nature", so he'd probably get a lot of weird looks but nothing on the level of notoriety people who get that tall get in say, real life (because it is Possible for real life people to get that tall, but documentaries are going to get made about them)

His devil fruit is, even more surprisingly, also something that could fit plausibly into trigun! i mean he'd probably want an explanation for it more plausible (sounding) than "i ate a weird fruit once", but even ignoring the things i'm deliberately leaving out for spoilers because i hope to con you into watching the anime at some point, there's some weird stuff. there's a guy who stops bullets by being really good at the saxophone. there's a different guy who kins doffy. (read: puppets people and makes them murder each other.) from what i've heard of the manga there's some kind of worm god just wandering around. rocinante could just be like "yeah i modulate the clap of my asscheeks to cancel out soundwaves of my choosing" and everyone would just have to roll with this. their only possible objection would be his notable lack of ass.

and he's one of the rare one piece characters who favors a gun, so his choice of weapon wouldn't stand out at all even if trigun didn't have midvalley the hornfreak wandering around. (aforementioned sax guy. admittedly the sax is also a gun so it's arguably not the best example but like. there's a guy with a saxophone that's also a gun. my point is that very little would be too weird.)

So he'd fit great in the setting! He'd be just another weirdo gunslinger with issues wandering from town to town. He might take a bit to adjust to the differences in technology, but both one piece and trigun are essentially the same flavor of "nominally 18th-19th century with occasional anachronistic sci-fi bullshit and/or modern tech thrown in at random" so mostly he'd just be adjusting to the anachronisms being in different spots. i do think plants would freak him out though, he doesn't have any real tolerance built up for eldritch stuff bc the eldritch stuff in one piece is better hidden and he doesn't know about it. (how to explain plants... uh. power plant that is also an angel in a womb and these are just a normal expected installation in all cities bc they're the only way to survive the deathworld trigun is set on. and from the outside they just look like giant lightbulbs.)

the question of how he'd fit in with the plot and cast is just as interesting, though.

see, vash the stampede, trigun's protagonist, is extremely similar to rocinante in a lot of ways. they're both gunslingers with evil brothers whose machinations they feel morally obligated to stop with an adopted parent they immensely respect and look up to drowning in oceans of angst they're too repressed to acknowledge. they're even both blond. but the specifics of their characters are different in ways that would make them repel like magnets.

vash is a hardcore pacifist. it's kind of his whole thing. his dead mom gave him a deep respect for the inherent value of human life and the conflict of his extreme dedication to avoiding lethal force and ideally avoiding even injuring anyone against antagonists doing their very best to make it the only option is literally the main conflict of the anime. rocinante, on the other hand, is not only a cop, he's a narc. he's a deeply morally grey character with a lot of blood on his hands and only moderate qualms about adding more. and despite this vash is the one between them who makes an actual attempt to kill his evil brother. (though the fact that he's doing this is mostly subtextual and the actual outcome of that conflict is much more complicated and interesting because EVERYONE SHOULD WATCH TRIGUN IT'S SO GOOD.)

so, arguably, they would hate each other.

but the conflict that would inevitably arise between them is startlingly similar in shape to the conflict that arises between vash and nicholas d. wolfwood, one of trigun's three deuteragonists, and vash and wolfwood are gay on a level second only to the likes of madoka magica and death note, so tbh a polycule is equally likely.

speaking of wolfwood, he and rocinante would get along like a house on fire and probably set several actual houses on fire. actually i think if i were to write a fic with this premise (thought exercise. don't get your hopes up i already have too many wips. but you know you could always watch trigun and write one yourself...) that's how i would justify rocinante intersecting with the plot. have him have some kind of one-off encounter with vash where they meet to fake out the readers before separating and then have rocinante run into wolfwood and have them somehow wind up sticking together for the rest of the story. and then vash would just be left trying to keep them both on no-murder flavored child leashes

alternatively, another trigun character rocinante might hit it off with is Milly Thompson. Milly Thompson and Meryl Stryfe are the last two major characters in Trigun, barring the antagonists. They're employees of an insurance agency trying to track down Vash and keep an eye on him so they can try to preemptively avert the chaos that follows in his wake (Yes, this is exactly as hilarious at it sounds.) and end up ultimately becoming close friends of his. (meryl has an implied het ship with vash and milly has one with wolfwood but i'm allergic to comphet so i usually choose not to acknowledge this. pretty good as comphet ships go though. just can't beat the vashwood ust.)

meryl is very spunky and no-nonsense and general plays straight man to the rest of the cast's antics, so while I don't know that she and rocinante would actively get along she'd probably be comfortable enough lumping him in with the existing pack of idiots she resents herself for being so fond of. milly is one of the aforementioned idiots (autistic flavor but done well and not actually dumb or pure comic relief. her hidden depths are rarely plumbed but the anime makes it very clear they exist and i find her endlessly fascinating) and is a cheerful ditz who i feel would see rocinante and adopt him like a maligned shelter dog in need of rehabilitation. i mean she wouldn't say this but she would think it. she is canonically a woman who enjoys a Project, why else would she like wolfwood so much