Bleach's last arc could habe been great had Kubo revealed the Shinigami's conspiracy that Ywhach only hinted at, but Kubo actually postponed it to a novel. That's not gonna happen with One Piece for sure though, Oda is not going to publish the Void Century as a textbook written by someone else
I still think this final war will shape into an Elbaf arc somehow. This is a pagan's Bible, "the" final pagan war is Ragnarök, and this whole Prince Loki stuff is way too weird. Like why get out of your way to even link this character's birthday with disaster? Why THAT name. Everyone knows Loki as a specific figure from Norse Mythology, Oda could have named the Prince Sven or Olaf or Harald or something had he wanted a generic name. Only Thor is more famous out of any special Norse figure.
There is also a gigantic tree - it can't bet left unexplained and we do miss the location of the last Road Poneglyph, which is what this final arc has to be about as that is what is missing to make Luffy the Pirate King.
I think Lodestar (Polaris, the North Star) atop the Elbaf tree (World Tree - - > Universe) could make a lot of sense. It has to be hidden as well, just not as hidden as Laugh Tale. Only Roger has reached it (or rather he was the first and is still the only one confirmed), which makes it unlikely to be a Sky Island as these are not 800years bulletproof secret. Lodestar is probably the second best hidden place in the One Piece World, and as such a perfect place to hide the last Poneglyph, and we know someone hid it, as its original location was Fishman Island. Whitebeard or Shanks seem the most reasonable culprits.
And with the reveal of "Ancient Giants" having been another project at Punk Hazard, Gigantification could very well be the "secret weapon". This entire plotline about Gigantification seems just too weird anyways as well, like I get the benefits of it, enormous strength and stuff. But there are also enormous downsides and I don't think any actual living person would really want to be a giant - so how can this story write this off as the dream of humanity, what is the purpose of it? There just has to be a payoff and I think the payoff is Ragnarök.
Anyways this whole "5 year stuff" is pr hype at its finest. There was a swap of editors some time ago and ever since then, One Piece has amped up its pr a lot. The confirmation is basically that they have been saying it for some time now - which means they are actually already stretching it. Don't put too much thought into it now as we are still going to be occupied by Wano arc for a while.