Loki will almost certainly join the SHs imo.
① Narrative with regards to the battle against the WG strongly indicates he will join the SHs given 1.) how much emphasis Oda's put on Rocks/Harald relationship and the emphasis on how significant Harald joining Rocks would've made on the war Rocks raged against the WG be it coz of Harald's strength or the legandary DF on Elbaf which according to Rocks would only reach it's full potential if Harald eats it. Rocks ultimately lost to the WG, but Luffy is bound to succeed where Rocks couldn't be it recruiting Harald or taking down the WG.
② Loki's characterization is a bit too unique for him to be just a one-arc character be it how unique he is even as an Ancient Giant or the fact that he was ostracized and is an outcast like most of the SHs. In this aspect Loki is much closer to the typical SH than a Yamato or a Carrot.
③ Hajurudin will always be in Loki's shadow if the latter remains on Elbaf and his dream to become the King of Elbaf is quite literally a pipe dream with Loki in the picture. The notion that a shadow as big as Loki will take a backseat to Hadrujin on Elbaf for the latter to shine is pie in the sky imo.
④ Loki's age is too much of a coincidence. Dude is the same age as Zoro and Sanji (63 / 3 = 21), so he'd fit pretty nicely in that M3 dynamic in a way that Jinbe cannot for instance. Plus the fact that he's only 21 is a strong indication that there's still room for growth for him so it is unlikely that he's as strong as a lot on here think he is even if he's a beast. Loki is mostly likely a power house but as has become pretty evident in recent chapters things aren't really what they seem with him (gone are the days when mofos thought he was gonna be antagonist of Elbaf

). There's a lot being said about him but what we do know is that he lost to Shanks 6 years ago. Luffy should at the very least be as strong as Shanks was back then which will make Loki weaker than him. That considered would you still say the gap between him and Zoro and Sanji is that big? so big that it cannot be offset but those two getting additional power ups this arc even? Keep in mind that Zoro and Sanji are relative to Luffy.