Some have suggested this is a retcon.
Some have complained that this wasn't foreshadowed.
It's not a retcon. This was long planned. Depending on the reading experience it may feel like a retcon, but this was not cooked up overnight.
"Oda said he doesn't know how Luffy will beat Kaido."
Oda says a lot of things. I'll leave it at that. I can simply confirm that this wasn't hatched overnight. Not liking/enjoying it are all valid. But it's not a retcon.
This isn't a stance to defend writing, Oda doesn't need anyone to do that for him and I'm not particularly interested in defending OP when it doesn't deserve it. This is simply pointing out blatant themes that were clearly and intentionally put into the series so that this kind of revelation could be considered plausible. If one doesn't like it due to taste, that's valid. But I'm sure you're going to see a slew of 'One Piece jumped the shark!' style of gotcha paper-thin opinion pieces based on emotion, not information.
Was the name 'Nika' thrown around for very long? No.
Does not adding a name to an element the author has broadcasted the importance of since the title of Chapter 1 mean an author did a poor job 'foreshadowing'? I'd say no, but maybe just having a 'name' for something that's been in front of them for 20+ years means something and I can accept that.
While we only recently put a name to the concept, the idea of the sun being a source of great importance has been around as long as the series. Technically even before it started.
・First and foremost, we have Oda coming from the 'Land of the Rising Sun' and he's a generation older than even myself. His association of our own Sun with his cultural identity is strong so seeing the Sun playing a role in this series isn't particularly unique. If you want to compare it to its 'on-time' peers, you'll note that even Naruto and Bleach gave sun powers to important characters. Simply put, if Oda has a chance to expand on the importance of the sun, unless he's positioned himself to create an aesthetically different shonen series that avoids references to actual Japan, you can bet that if presented an opportunity to touch on Japanese culture, the tenets of their cultural identity is going to peak through. All of this is really just to say, given his age, his views, his peers, when he began writing, and the story he's telling, there was a high probability of the Sun being 'powerful' at least and 'important' at most.
・Then there's the title of Chapter 1 and Volume 1: Romance Dawn.
A name so important to the series, it was the name he used for the series when it was proto-One Piece. From the get-go, the series is telegraphing the importance of a new dawn. If you boil One Piece down into MAJOR encompassing themes, that does the job so well it's almost unfair how good they sound sound together. The 'Romance' encourages everything 'shonen' about the series (friendship, effort, victory) along with adventure while 'dawn' tells us that it's a story of a new age. In retrospect, we've come to learn that this concept of a 'dawn' is not only signalling the new age to the readers, but it's also something important to 'in-the-know' characters in the world as well.
・An extension of the previous point, the 'do-n' sound effect Oda is such a fan of, is literally the same way to write the English pronunciation of 'dawn'.
・ https://one-piece.com/special/greg/detail/20160504_0557.html?l=en The tldr is that not only the sun proper, but a strikingly similar depiction of it has been incorporated in the flag of Alabasta (a country which rejected membership in the WG that Imu is obviously pining over) as well as being included in the Kouzuki clan mark. Touching on anything from Wano is still skipping ahead but the important thing is it's there.
・The Thousand Sunny. Judging from all this use of 'sun' imagery, on a personal level outside this story, the sun represents something special to Oda which shouldn't be surprising. It's a source of pride in identity. Another symbol that's important to Oda? The lion. The lion represents a type of regality to him that while I can't go into specifics, is something that he enjoys to express in real life. I believe it's also why we haven't seen a lion in the series yet, because of the importance he assigns to it as a symbol of regal power. The figurehead of Sunny combines both of these elements into one as a lion that also looks like a shining sun. I can all but guarantee that this choice was made knowing what Luffy would Awaken.
・The concept of the 'sun' is a foreign one to the fish and merpeople living below the sea. That's why the 'Sun' in the Sun Pirates is written in Katakana, rather than kanji, because it's an abstract concept. Is it a coincidence that Joy Boy wrote an apology to them after they remain living underneath the waves of the sea but clearly have a desire to bask in the glow of a real sun? The revelation of Joy Boy's existence & failure and the existence of the Sun Pirates alongside Orihime's goal to bring the fish/merpeople to the surface was our first hint that these unrelated concepts would be related.
・https://one-piece.com/special/greg/detail/20180829_0931.html?l=en tldr: now we have Luffy being directly linked to the sun in coyly presented forms. Sunflowers (also very important symbols in Japan), a lion design that oop also looks like a sun, then literal marks of the sun identical to ones on the Alabasta flag and Kozuki clan mark? Not only that, but even in 'throwaway' color spreads from years gone by?!
・Pedro brings up talk of the 'dawn' and we finally learn that characters are aware of the so-called 'dawn' in-universe and are anticipating it.
・The former daimyo clans of Wano (except the Kurozumi) all bear 'moon' in the name and yet the Kouzuki clan mark clearly displays the mark of the sun in their mark. This suggests a strong link between both the Sun and the Moon. Further along, King mentions the importance of Joy Boy to the Lunarians suggesting that Nika is very important to the Lunarians as well.
・Hyogoro called Luffy the 明王 myouou which people have been throwing around as 'wisdom king' thanks to armchair wikipedia scholars, but not many mentioned that the name proper actually directly translates from kanji as 'Lord of Light'.
I'm sure that those re-reading the series now will come across more and more of these observations. These are just a few that I remember off-hand. For instance, will we learn that Mamudein or other characters/events mentioned by Robin were actually related to Nika? Since Nika proper seems to be an original amalgamation of real-world ideas/themes, it stands to reason that cultures from around the OP world would have different ways of interpreting this figure.
And there's very little doubt in my mind that this 'mythical' figure is actually as 'mythical' as it seems.
We know very little and the design is lacking some important features that help it tie together nicely, but one of my ideas is that DF's were made from natural phenomena or people/animals that were the result of Lunarian experimentation. If Nika was a Lunarian (which I don't know about given current lack of wings) who consumed a GGF and then Nika himself was turned into a DF, that could explain how a Paramecia could be merged with a Zoan. That's just spitballing but it's based on the concept that GGF's Paramecia roots have not been rejected but it is also carries Zoan traits, so how do we fit that puzzle together? Lineage factor may be the key there.
Either way, it makes a lot of sense why Oda has been holding off Vegapunk.
I hope that helps express why readers should've been prepped for heavy-duty mythology related to the Sun. I first mentioned the importance of the moon somewhere around 2008-2010 and over the years people seemed to bite into that and get tunnel vision, but the importance of the sun has been here the whole time if you were looking for it!
One issue I am grappling with is… One of the core theme of this series is freedom. Luffy says himself, that being pirate kings means to him is the ability to do whatever he wants.
Therefore, the juxtaposition of Luffy and his entire fruit being kinda predestined leaves a bad taste in the mouth.
Luffy ate the fruit when he was 6/7. So more than half his life. We don't know much of Luffy before that time. Given it is more or less confirmed that DFs have a will of their own, it beggars the question that, how much of Luffy is actually himself, and how much of him is subconscious DF manipulation?
Luffy was, again, just 7. So, the DF should have an oversized influence on him!