What is a plot that you think went effectively nowhere in Turn, or wasn't realized properly? For example, I that Robert Rogers starts feel auxiliary in the third season onward.
Oh honey, how long do you have?? There were legitimately so many plot threads left dangling that could've really had the potential for something really good!
For one, Nathan fucking Hale! He gets what I consider a passing mention, almost entirely in service of creating a parallel for the death of John Andre in S3 and giving Ben some anti-Brit angst. That's it. The real Nathan Hale's death was a huge deal in the Revolutionary War and impacted the way that both sides went forward with spy work -- if you're gonna make a show about Washington's most famous spy ring, and Benjamin Tallmadge is (debatably) going to be one of your leads, you can spare more than a passing mention for his close friend Nathan Hale.
For another thing, the sewing circle that Mary and Anna attend all of twice in S1! They introduce it as a way for Mary to accidentally spill the news about the gravestones, and then poof! Never a mention of it again! Is this a spy show or what?? Let the sewing circle become its own potential information exchange circle -- heck, you can even do the 'jealous two-faced bitches all around' thing if you wanna!
Or how about this: the entirety of Abigail's story?? Like, to this day we don't know who Agent 355 was. You make the choice to envision her as an educated enslaved woman. Okay, great -- post of story potential there. But if you're going to do that, you have to commit. If you want her to be a dedicated Patriot spy, you gotta give her good reasons for wanting to do that when her obligations run out. And if you don't think you can do that, then don't make her Agent 355. Just don't. Write her as a white woman, and let Abigail be doing a bunch of dangerous shit because her actual enslaver is forcing her to. Oh, but that would make the worst parts of slavery really prominent, and we don't want to do that, do we??
But I think the abandoned or snipped threads that tend to piss me off the most have to do with Ben's character development. For example, do you remember the scene in S4 where Washington tells Ben the truth about Nathan Hale's Fake Last Words? Did you know there was a deleted scene filmed for the first episode of S3 that has all the same character beats, down to some of the same dialogue? Ben could have learned this lesson about spycraft and honor a good season and a half earlier!
Or how about the time in S3 in 'Many Mickles Make a Muckle' when Billy drops a line that the cinematic language treats as Super Fucking Significant? Ben's expression changes while the camera zooms in and the music gets all Ominous -- like, 'ooh, is Ben maybe realizing something is up with the Arnolds?' And the first time I saw that, I was like, 'ooh! Ben's gonna Detect Something with his Spycraft!' But nothing came of it! Nothing!!
But the Ben plot thread that had the most potential, the handling of which pissed me off the most? The Sarah Livingston arc.
Now, I wanna say up front that I'm not crazy about the way that I've seen a lot of people in the Turn fandom react to Sarah and the Sarah/Ben association. Like, I get being queer and projecting onto him, or just straight up thirsting for him, but the level of vitriol I've directed toward a female character for having sex with a man they felt entitled to really rubs me the wrong way.
Having said that, the way the show itself treats Sarah is shameful. They throw her at Ben for all of two episodes, and then fridge her in the worst way (a r*pe-murder scene? Fucking really?!) just to have Ben Angst! Like, y'all had this golden opportunity to have Benjamin Tallmadge, this preacher's boy, really question his own morality, and the relative morality of his cause -- this opportunity to give some real characterization to a Loyalist who has real cause to hate the Patriots, a woman no less -- and y'all squander BOTH of these things??