Hmm, the question of the thread is more general than i thought. Politics is a lot of things. Who should govern, what political system is best. What is the best governing. What is moral? Ethical? Who dictate it? What is the best way to achieve something? Who enforce moral? It is more than just a simple Yamato's gender. When you ask what is the deeper political theme. I really like to go back to Otohime and Fisher Tiger. They had some what similar concern, but how do they achieve their objective. And which way is the best? And what is the story structure aka Oda had to say about each of their ways.
When you have geopolitics, if you gonna go something like "4 Emperors ruling territories." What the heck is that meant? What is so deep about it?
But if you said well the 4 Emperors are the lawless part of the world as opposed to the WG. Now that is more interesting. What made those places lawless? Big Mom and Kaido had rules. They had laws and power hierarchy. That is no more moral/amoral compare to the world government. This is very much like said something like an Israel post WW2 or a current Taiwan right now. An entity that is acknowledge or not acknowledge by other countries. Even then though, the WG do acknowledge the Yonko.
And then revolution, is that all good? Alabasta had a revolution. Disorder, overthrowing the established hierarchy isn't necessary great always. The Arab Spring was cheer on by the west. And since then, destablizing and throwing many countries into chaos, making many lives much worst off than before.
To be when you talk about deeper politics, I am thinking about Hody saying nothing was done to him when talking about racial hatred. Or Doflamingo's family being murder for doing what the father think was right. Heck even the parents in Wano trying to kill their children due to not enough food. Who can have kids is political nature (see China's 1 child policy). How resources are distribute is political. Should you have children when the world around you going to shit. Is that moral to bring a life into the world full of suffering. That can too be political.
If you talk about the marine, then if the institution corrupted from the root. How should one enact changes? Work within the system? Outside of the system? Or symbol meant something at the end of the day, and ignore the larger politics and just uphold this image of justice.
"Might be that this political aspect was always there, but somehow not in the focus." Maybe you just didn't think about it. Let's talk Luffy/Ace's fb. When you burn the undesirable people along with the trashes for the appearance of nicer society. That is a political implication. What about this? Should tax dollars go to making the homeless uncomfortable and they just leave an area. Out of sight, out of mind, rather than helping them. That is very much politics.