Loki and Shirahoshi hints
In no particular order...
Neptune and Harald talk (and agree) on their betrothal before Shirahoshi is even born. (c. 1167)
2. Loki is tied to an Adam tree. Shirahoshi lives among the roots of Eve tree. (c.1131) (c. 612) Both describe themselves as being imprisioned.
3. The Elbaph giants talk about how they'd love to see mermaids. (c. 1136)
4. Later Jinbei compares Loki's size to a Neptunian. Shirahoshi can command Neptunians. (c. 1178)
5. The conversation between Loki and Mosa (hairy/shaggy, which in itself is another hint since there's a recurring joke that Neptune's daughter would be hairy like him) suggests something happened to Mosa that perfectly fits what happened to Shirahoshi at Mary Geoise.
He also complains about his chains, while the panels where Shirahoshi was attacked had her in chains too.
(c. 907 - first attempt at kidnapping) (c. 1084 - they came back again, like Loki said)
6. The Noah arc is at the sea forest, needing repairs. The Neptunians suggest a certain "them" will be able to fix it. (c. 648)
Meanwhile the Elbaph royal family seems to be able to command a group of hardy giant shipwrights called the Galleila. Harald is gone, but as his heir, Loki should be able to do it. (c. 1154) (c. 1166)
Furthermore, it's possible we have already seen the Galleila frozen at Punk Hazard. (c. 660)
Loki just so happens to have ice powers, thanks to Ragnir. He could unfreeze the giants and go to Fishman Island to repair Noah. (c. 1179) (c. 1187)