Tbh it depends on how it's done. He makes it seem like the structure won't be the same (i.e. I will draw something never seen before). Of course that's an exaggeration, but Oda's outline for a final major arc has been around for 2 decades. The original plan was for the final arc to be a 5th of the full storyline:
From APForums, look at the fucking dates on these lol:
The whole world will be thrown into turmoil in the final arc."γAsahi Evening Newspaper (26 November 1999)
Questioner: Many are looking forward to what happens next in One Piece could you give us a message for them?
Oda:: One thing that keeps me motivated to keep drawing One Piece is that I want to draw the last chapter. The last chapter is amazing! I imagine a final chapter that will make Marinefold arc look like nothing!
Questioner: Seriously?!
Oda:: The Marinefold arc really felt like a side trip to me. I didn't expect it to be so popular. So I don't know how readers will take this one. But the thing I think is interesting will be in that last chapter. Until then I would like as many of the fans as possible to stick with me. Eternal Log (2012)
http://apforums.net/showthread.php?t=35262
"I was going to finish ONE PIECE within 5 years.
Every crew joins before Grand Line. (1- 1.5 year)
Great adventure (3 years)
Final arc (1 year)
If hes maintaining. That type of structure (outside of when the crew joins), then we still haven't left the "great adventure" portion of the story.
This is why I dont believe his 5 years left comment has to do with the final arc. He mentions it in relation to "Luffy's journey to one piece" , not serialization. This is why it makes sense he calls Marineford a side story, because it's not progress on Luffy's journey. The Final War technically won't be either.
Tldr this shit is going to be WAY bigger than Wano