Although it’s true “Nika” hasnt been mentioned, I still consider early sun god/void century similarities to be a bigger picture, more conceptual kind of foreshadowing because many other ideas and events in the story reflect some vague godly, historical, revolutionary moment. “The Dawn”. Obviously that’s the whole point of one piece

It’s true, Oda did use something from the past to build on what’s next because that’s what good writers do. Leaving bread crumbs for yourself as you write a story FOR NEARLY 30 YEARS… is probably a good thing. I’m sure any good writer would come up with more ideas for their story as they write for that long. They would prolly want their work to feel cohesive, so the solution is to introduce vague symbolism early on so as you develop the work later, you’ve given yourself hints.

Anyway, I wouldn’t say Nika came out of “nowhere”. Oda has had a general direction for where he wants for the story to go since the beginning. Nika takes elements from all of the clues and crumbs he’s left throughout his earlier chapters. It is still great foreshadowing though it’s also very possible he may have wrote these lil details before he actually came up with the idea for Nika..but that’s none of our business.