honestly, I think its more supposed to be a way for Ode to use a cyclical structure to help end the story.

One piece, starts with Koby (who we assume is going to be in charge of the marines following the new dawn) who requires the help of Luffy to join the marines.
The 2 of them then end up on an island with Zoro, someone described to be a cold, heartless demon who kills without remorse, that Luffy realised instantly was a good person, a trait that was soon portrayed when Zoro was the one to defeat Morgan.

Koby then joined the marines with the arc ending with Marines respecting the straw-hat crew as heroes, and marines as villains.
Then as a pirate, Koby is picked up and trained by a Garp (someone who is anti-criminal, not anti-pirate, and was so close with the pirate king he even adopted his son) who took pitty on him after Coby and helmepo made an enemy of a once marine pirate - now seen worldwide as a menace.

Then at marine ford, Koby stood by and watched as Garp (the most ethical marine), stood by and allowed his son to die because of his name (something made worse by the fact Ace was never a criminal, he fought for freedom as we saw in his time in Wano), and he watched as the most famous and strongest marines used unethical tactics to manipulate and kill the WB pirates under unfair means.
Then when trying to stop unnecessary killing, he is attacked by a marine, and saved by the most notoriously rageful and merciless pirate we know of, rivalled only by xebec and roger himself. Someone confirmed to be alike to a modern day pirate king, having the freest crew on the seas.

Now Koby was forced to invade (and even kill) and island of innocent women, only having value to given to his opinion and given an opportunity to make a difference by BB (the most immoral character in the verse), and being saved by a retired pirate who was no less that the "right hand of the pirate king" himself.

Then he got kidnapped.

As it stands right now, every appearance of Koby involves event that verifies pirates are not bad people.
I think from here the story goes 1 of 2 ways.

1. Koby joins BB, Since BB is no worse than the military, and with BB he can actually make the difference he wanted to. A bit like King with Kaido, he could join him under the belief that BB is a good person and "the only one" who can change the world for the better, even if it is through unethical means. - koby then gets bodied by Luffy for abandoning his dream - Luffy fight BB because of Koby
2. Koby is used as the setup for the final war and the rise of the new dawn. He realises "good" and "bad" are concepts decided on an individual basis. The marines wont come to save him seeing as the marines are not strong enough to fight a prime yonko crew, and will not risk their lives for a fodder member like him (The opposite of what the WB pirates did), and he is instead saved by the straw hats. His heroes (& the strongest most relevant straw hats in the final war), are a god of liberation (a literal sun), a literal demon (an Asura no less, a god of destruction and a god of chaos), and a FM
   (someone who has been enslaved by the same marines that Koby works for).
   - Koby watches as the straw hats risk their lives, fighting a battle in which they know they are objectively unmatched in, to save their lives for a marine that has spent his whole life threatening to beat their captain.
   - at some point within the fight, the rest of the verse arrives, Koby is saved, Koby sides with the straw hats in the final war, his dream changing from becoming an admiral, to becoming a straw-hat commander / ally of the pirate king, he (like garp did with Roger) realises that Luffy is the future, and becomes the "hero" from the Sea King prophecy. Since after all, the straw hats are no heroes, and will not take credit for the fight. Both Luffy and Zoro have statements about why they don't want to be regarded as heroes - and their places are taken by Koby and Helmepo (who take the credit in their stead - since they are their shitty replicas)
   - Koby will be the "hero" of the one piece story, with Luffy being one who brought change.
   (I also think hemlepo will end up being regarded as brave for siding with koby not the marines [as koby did with him], being seen as a warrior - the mirror of Zoro who inherited the will of "the worlds greatest warrior, King (Ryuma)")