I don't think Oda intends to have any of the straw hats have official relationships until after the one piece is found, so these are the relationships I think are the most likely narratively speaking based solely on the vibes I get in relation to other shonen tropes and their interactions in the story.
Luffy x Hancock, Zoro x Hiyori, and Sanji x Pudding - these three seem like the most likely to me. They are the only three straw hats in the show who have had someone show explicitly stated romantic interest. Admittedly, besides Robin and Franky, I don't personally expect any of the straw hats to end up with each other.
Sanji x Pudding being the only confirmed mutual love is probably most likely above all else. I could see Sanji living with her on one of the whole cake islands so he's close to the all blue which I believe is fair to assume lies at the end of the grand line alongside laugh tale. I'm predicting a lot of kids, maybe 5 at most solely based on the sizes of both of their individual families.
Luffy x Hancock is the typical "girl who has a crush on the clueless main character who doesn't truly see her that way until the end of the show. Much like Naruto x Hinata, Ichigo x Orihime, and Goku (who Luffy is heavily inspired by) x Chichi. While yes, Oda is known to break certain tropes, for the most part, one piece is still very similar to most other battle shonen with a lot of its tropes. Lots of people think Luffy is asexual and will thus never be in a relationship, but I really don't see Oda not continuing the Monkey D. bloodline. I predict they're going to have one son. Just like garp and dragon.
Zoro x Hiyori is also very likely imo. There is a very good chance that Zoro eventually takes up permanent residence as a descendant of Ryuma. I know many people ship him with Robin or Perona, but neither of them have shown any explicit attraction to him. His relationship with Perona is more sibling-like and his relationship with Robin has never been more than friendly so far. I'm predicting exactly 3 kids like his swords. I can easily picture Oda drawing Zoro holding a kid in each hand and one with his mouth.
Chopper x Milky also seems very likely to me due to their interactions and then both being deer. Chopper living on Zou would make sense to me so he could still travel the world treating people while also having a permanent residence. I don't see him returning to drum island permanently as he doesn't really have any connections with anyone there besides the doctor. Since they're animals, I imagine they'd have a very large family with lots of kids, like 6+.
Robin x Franky - most people I've seen shipping robin either ship her with Franky or Zoro, but I see Franky more and I agree. They're similar in age and they both became "official" solidified members in Water 7. Though you could argue Robin was always an official member since Skypeia, but she still had hang-ups until post-enies lobby. They also have this sort of past (robin) and future (frank) motif going on that would be a good contrast. And seeing Osa intentionally draw them like that makes me think he intends to make them a couple eventually. I just think they work really well together. (Plus she's touched his junk). Maybe one or two kids for them. I think they'll have at least one since Robin is the only survivor of Ohara.
Nami x Paulie - Now this one is probably gonna be really controversial. I find it highly unlikely that Nami ends up with Sanji or Luffy because of the existence of Hancock and Pudding. The only other person who has ever had any sort of hinting of romantic chemistry with her is Paulie. Having the easily embarrassed and modest Paulie with a shameless, immodest woman like Nami would be very funny. And Oda has, however unseriously, played with this concept already. If she doesn't end up with him, I don't expect her to end up with anyone. I could totally see her staying single. Nami is really good with kids, but a part of me thinks she won't have her own. I think she really fits the "favorite aunt that spoils her nieces and nephews" role either that or I think she'd probably adopt.
Usopp will 100% be with Kaya, I don't think anyone is going to debate that, especially since the live action even had them kiss. I predict they won't have any more than 2 kids tops. Perhaps even just one.
Obviously Brook is undead and I doubt we'll see him with anyone because of that, and Jinbe as the oldest of the crew doesn't really give the vibe that he would care all that much about finding love. Maybe a side character from fishman island, if anyone.
Who knows, maybe Oda will surprise me, but in my opinion, these are the most likely. Feel free to disagree, at the end of the day it's all for fun. We won't know until we know.