Let’s start with the Monster Trio. We had an entire arc primarily focused upon Sanji, and a good portion of it was dominated by his interactions and relationship with Pudding.
Starting out as necessity, only for him to know her better and vice versa and whatnot. They (un)officially married, and parted on terms which seemed like Sanji might just return to her after everything’s said and done. Looked very plausible.
Then came Wano and we abandoned the usual trend of Luffy running into the “Princess” and had Zoro save her instead. Nothing indicates there are unrequited feelings on Hiyori’s side, but she was impressed by his strength, and has had so many teasing moments (or, hell, beautiful ones. That shrine scene was amazing) with him.
Not to mention, Zoro seems exactly the type of person she’d fall for; a swordsman who is incredibly strong and honorable, and might just end up as her country’s savior (or at least partially). She even gave him her father’s meito.
Then there’s Luffy and Hancock. Nothing to say about them- seems like Goku&Chichi thing.
That aside, there’s dangerously accurate pattern that these three follow tbh. Tragic past, impressed by non-apperance-based characteristic of protagonist, and most importantly…. their hair color.
While not exactly same, they’re very similar shades of base general coloring. Hell, teal color doesn’t even /fit/ Hiyori’s character design, which screams black, but Oda intentionally did it anyway. Meanwhile Hancock and Luffy both have black, similar to the other pairs.
Then there’s Zou, where Chopper ran into that Deer girl he seemed enamoured with. Not only that (alongside the fact that Zou, as a place full of walking and talking animals is perfect retirement zone for Chopper), it was also hinted in an SBS that Chopper might pursue her someday.
Not much for the rest, but… yeah. It’d be pretty cool and so far, portrayal-wise, very plausible for Sanji to return to Pudding after everything (defeating her mom, too), Zoro to find retirement within Wano as the country’s hero and the best swordsman across the entire world, Chopper to find himself in Zou alongside rest of the kin after everything, and for Luffy, if he ever stops his adventure prior to dying, to end up at Boa’s place where there’s…. a lot of food, I think.
Oh, and the obvious, Usopp returning to Kaya. While nothing is hinted in Manga for these guys, I think the most plausible retirement for Brook would probably be the entire concert thing, like within the time-skip. Franky is probably going to be remain with the ship and do… not sure what. Can’t say about Nami and Robin.
*Theory by Shanal9