Yeah, I think Oda was trying something during the Yonko Saga. And I'm not sure if we really have enough information to say exactly what.
I think a lot of the Yonko Saga's problems is that Oda started focusing a lot more on the overarching meta-story, rather than the individual stories of the individual arcs. He became more focused on the metaphor and the message of it all, rather than the stories involved to get there.
I think it's notable that Post-Timeskip, all the villains of every arc have had more sympathetic motivations than the villains Pre-Timeskip, in the arcs themselves. Pre-Timeskip, all the villains just seemed to want power or glory. But, Post-Timeskip, Oda's even giving the Pre-Timeskip villains more fleshed out motivations and backstories, retroactively. The villains all still went about their goals in the "wrong way", but their ultimate goals were a lot more justifiable.
Hody Jones just wanted to end the slavery of his people...by enlsaving the slavers.
Caesar Clown wanted to unlock the secret of Devil Fruits and the past...by experimenting on children.
Doflamingo just wanted to go back to his childhood home...by tricking a entire country into thinking he's their hero.
Big Mom wanted to unite all races...by forcing them to join her family against their will.
Kaido wanted all people to be equal and to fulfill a prophecy that would bring about worldwide peace...by creating a endless war where might makes right and trying to force the "chosen one" to come and fight him.
In a way, they're kind of all "Failed Joyboys". Trying to change the entire world to suit their own dreams. It's just that they're making everyone else miserable in the process.
So, I think Oda was trying to show all the ways that Luffy could go wrong. Just in order to show us why Luffy's way will end up being the "right way".
The second thing Oda seemed to want to do with the Yonko Saga seemed to be to show us the "origin" of Devil Fruits. By showing us, in pretty decent detail, how SMILE Fruits were created and why.
Many islands in the Yonko Saga had something to do with SMILE production. Punk Hazard showed us where SAD was created. Dressrosa showed us how the SAD was turned into SMILE Fruits by giving it to trees. And Wano showed us how those SMILE Fruits ended up being used.
This will probably all end up paralleling how Devil Fruits were created and distributed during the Void Century. It was probably a worldwide production, that involved many locations around the world. Which likely had similar drawbacks for the commoners of those lands, as we saw in the islands involved in SMILE production.
The third thing that Oda may be doing, and this is probably the most "odd" idea I have...is that Oda is basically showing us the history of the World in reverse.
I think Luffy is basically undoing all the mistakes and crimes of the past, one island at a time. Just in reverse order. So, Return to Sabaody would represent the time closest to the present day. And Laugh Tale would represent the earliest point in the timeline, where the first "wrong" was commitment that needs to be undone. Oda is slowly showing us the past, having each arc represent a different time period, and going a little farther back in the timeline every time.
Every arc has been about undoing what once went wrong. So, when Luffy "fixes" something in one Kingdom, I think we can assume that same thing basically "broke" when this originally happened, long ago. We're seeing everything get fixed. But, originally, things went the wrong way. So, in the past, everything broke and went wrong. While we're seeing things get better in the main story, originally they got a little worse all the time.
In other words, the history of the World would look a little something like this:
Era Zero: “Laugh Tale”
-We don’t know anything about it, but this is where everything started. It’s possible a Treasure lands on the World from outer space or something.
Era One: “Lodestar”
-Again, we don’t know anything about this place. But, I think we can assume some game-changing element or Treasure was discovered. Something never seen before that everyone wants? This could represent the discovery of the Mother Flame or Forbidden Sun.
Era Two: “Elbaph”
-Our Arc began with an island at peace, and ended with a war, so…
-In a primitive land, war breaks out. People fight amongst each other. Possibly to gain the Treasure found during the “Year of Lodestar”. Eventually, the fighting ends leading us to…
Era Three: “Egghead”
-Our Arc began with a technologically advanced island that got destroyed, so…
-The Treasure allows rapid scientific advancement. New technology is created practically overnight. This could be the rise of the First World.
Era Four: “Wano”
-Our Arc began with people being oppressed in pollution and hunger, and ended with the Ruler being overthrown and the people rejoicing, so…
-The World becomes a land of excess and pollution. Dragons roam the land. A Ruler comes to power and oppresses the people. People are enslaved. People begin sealing themselves away from each other, forming walls. Nika/Joyboy’s lowest point, as a opposite to Luffy’s peak. It’s possible this represents the end of the First World, where it gets destroyed by the Hellflame Serpent. Kaido and Orochi may represent the two "sides" of the D Clan? This may be when the Monkey and Davy Families may have started their rivalry.
Era Five: “Levely”
-Our Arc has everyone’s preparing for a big war, so..
-The World Leaders come together for the first time to discuss trade. Possibly the actions of the 20 Kings start to bring people together. This may be the start of the Second World.
Era Six: “Whole Cake”
-Our Arc begins with a land of excess about to celebrate a wedding, and end with the Ruler going missing, so…
-The Rulers reveals themselves. The World is broken up into territories (the opposite of a wedding?). More excess. More madness. The Ancient Kingdom creates wonders unforeseen before. Big Mom may represent Imu, as a survivor of the First World during "Wano", who tried to go off and create their own perfect Kingdom after the First World failed. Creating a "sanctuary" for her "family" the Celestial Dragons, like a proto-Mariejois.
Era Seven: “Zou”
-Our Arc sees a moving island whose people agree to join a alliance, so…
-Refugees and slaves try to escape from their oppressors. Trying to form their own colonies and areas away from the Ancient Kingdom’s influence.
Era Eight: “Dressrossa”
-Our Arc sees a peaceful kingdom full of toys learn about the truth of their land, and become equals with each other, so…
-Slaves are turned into “toys”. Slaves are sent to work Underground. The truth of history begins to be hidden. The divide between rich and poor grows greater. It’s possible this is the start of the Second World, where Devil Fruits are born, represented by Dressrosa being where SMILE Fruits are created. Imu’s rise to power may parallel Doflamingo’s own rise through spreading lies.
Era Nine: “Punk Hazard”
-Our Arc saw a island that was destroyed and slaves were freed, so…
-The Ancient Kingdom is destroyed. Everything burns or freezes. Slavery becomes commonplace. Devil Fruits are unleashed onto the world by accident. This would be the end of the Second World.
Era Ten: “Fishman Island”
-Our Arc saw a peaceful land almost tear itself apart due to racism, and clinging to the past, so…
-People segregate themselves into different areas. The “lower realm” and “Mariejois” is formed. Racism becomes more pronounced after the destruction of the Ancient Kingdom. This would be the beginning of the Third World.
Era Eleven: “Return to Sabaody”
-Our Arc saw old friends come together to fight against their dark reflections, and the Government, so…
-Everyone splits up on different islands. No one communicates with each other anymore. “False Heroes” rise, as a metaphor for the Fake Strawhats. The World Government rises and takes over.
That would bring us more or less to the present day.
So...it COULD be possible that the Pre-Timeskip arcs actually parallel things that will happen in the future. If I'm at all on the right track with this. I've tried a hypothetical scenario with that, too. But, I don't think it works as well.
So, I think Oda has had a structure in mind for the Yonko Saga and beyond into the Final Saga. It's just so much bigger than you'd expect, that it's easy not to see it.
Ultimately, I think Oda got so caught up in telling this epic, that he lost sight of some of the individual details during the individual arcs themselves. He's been so focused on the "Forest" that he lost sight of the "Trees", basically. Telling a story that works backwards and forwards isn't easy. And that might explain some of the awkwardness to this huge saga.
But, I think once we learn more about the Void Century and the secrets of the World, all this might snap into place. And we'll be able to more appreciate what Oda's been trying to do here.
The Yonko Saga definitely had it's ups and downs. But, I think the "downs" were in service to the larger meta-story that Oda's trying to construct.
...Or maybe I'm giving Oda WAY too much credit. But, time will tell.