The Aftermath of Coby vs. Hancock
One of the key outcomes of the Reverie was the abolition of the Seven Warlords (Shichibukai) system, an event that has been teased since Dressrosa. Perhaps one of the more surprising outcomes is that the Marines have decided to go straight for the heads of the pirates that once served them. The most exciting fight for us as fans, though, was the teaser of Coby vs. Boa Hancock. The scared kid from East Blue versus the revered Pirate Empress. A guy who's been on his journey as long as Luffy has been on his, versus the woman who loves Luffy more than anyone (pls don't kill me Bartolomeo). The outcome of that fight is one that is being hotly debated, and this theory is going to go over some possible outcome scenarios for that fight, and work out how they'll affect the "endgame" of the One Piece story.
Before we get into these scenarios, however, there are a set of conditions that the outcome to this fight needs to "meet" for it to be viable.
1. Boa Hancock has to be able to team up with the Straw Hats.
For this condition to be met, I think it's unlikely that this fight ends with Boa in Impel Down. This doesn't mean that the fight doesn't end with her capture, though. More on that later.
2. Coby and SWORD have to be able to team up with the Straw Hats.
A big call, I know, but one that we currently have precedent for, as of Chapter 993. An incident I refer to a lot in theories is that of God Valley. This incident sets precedent for the idea of Pirates and Marines working side by side. Secondly, X Drake, a SWORD member, is teaming up with Luffy in Wano. Through his flashbacks, we know Coby has been talking about Luffy in a very positive light, and that further raises the potential for them to once again fight on the same side, just like they did all those years ago back in Shells Town. Luffy and Coby are friends despite not being on the "same side", and the conclusion of their interactions hinted at an ideological divide within the Marines (all pirate bad vs. some pirate good). I think for Luffy to become the Pirate King, he will have to fight alongside a Marine faction, and right the "wrongs" done to Roger when his involvement in the God Valley incident were covered up. To truly achieve freedom for all pirates (freedom is a major aspect of what being the Pirate King is to Luffy, and I think it's established at this point that his actions will end up bringing about major change), they need to be able to exist in a world where they're not generally perceived as criminals, and to do that he needs a group of Marines, the "good guys", to support him. Obviously, guys like Garp and Smoker could fill that role, but because of how this story was set up to us at the very beginning, I think the Marine to truly bring about that change will be Coby. And speaking of Coby...
3. Coby has to be in a position to be promoted to Marine Admiral by the series epilogue.
Okay so, the first thing I will say is that Coby will not hold the rank of Marine Admiral during the final stretches of the series. Only after Luffy becomes Pirate King will Coby become an Admiral. Both Luffy and Coby are well on track to achieving their dreams, but Luffy has always been portrayed to be several steps ahead, despite Coby's rapid growth and rise up the ranks. Of course, Coby will be in pole position to be an Admiral just by being on the "winning" side of whatever final war happens. However, he will need to have strength feats to back up the good words that many will undoubtedly put in - Luffy, Garp, maybe even Shanks if he makes it to the end of the series. Capturing Boa Hancock is a big test for him, especially because we've seen how dangerous she is and Coby's rank would imply he's not close to her level. However, we know he's full to the brim with both potential and resolve, and there's very little doubt in my mind that a character whose dream was so clearly established almost as early as Luffy's dream was will see that dream come to fruition.
To summarise so far:
> Both Hancock and the Kuja Pirates, and Coby and SWORD, will ally with Luffy during the final war.
> Coby needs to cement his status as a "Hero" of the Marines, and be seen as a peer to Luffy.
These sets us up for a very interesting endgame...
Okay so, it's theory crafting time now. Here's the sequence of events that I'm proposing:
> The conclusion of Wano results in both Kaido and Big Mom falling at the hands of Luffy and the alliance. Luffy, being the biggest name and the figurehead of said alliance, becomes a "legitimate" Emperor of the Sea (rather than being an Emperor via tabloid hype).
> Coby and SWORD successfully subdue the Kuja Pirates, with Coby being a key player in the defeat of Boa Hancock. This moment for Coby is the equivalent to Luffy's "Crocodile moment" in Alabasta.
> Boa Hancock is being held captive on a Marine ship, and is talking to Coby. As they're talking, a SWORD member bursts into the room, bearing news from X Drake. Straw Hat Luffy has taken down both Kaido and Big Mom in Wano Country. This is the moment where the two realise they have a common ally.
> The aftermath of Wano leaves Luffy in a dangerous position. With news of his accomplishments spreading, another Emperor makes his move - Blackbeard. His move is to go for Luffy's head. This is where we really find out the extent of what Blackbeard's been up to in the New World - he's likely got the backing of the Level 6 Impel Down inmates he's freed, along with other powerful pirates that share his worldview. The play Blackbeard is about to make is the one that will give him a direct path to being Pirate King.
> SWORD, who have the Kuja Pirates in tow, head to pick up Drake, who is likely waiting on Wano to ensure that Kaido/Big Mom's capture moves smoothly. After picking up Drake, their plan was to head to Impel Down. However, that plan is halted when news that the Blackbeard Pirates have attacked the Straw Hats. With both Coby and Boa being concerned for Luffy's wellbeing, an agreement is made between SWORD and the Kuja Pirates to head to the location of Luffy and Blackbeard's battle (which will either be the location of the Fourth Road Poneglyph, or a place where the Straw Hats can learn of its location). One of the terms of this alliance is that Boa Hancock and crew are allowed to walk free.
> The Straw Hats, SWORD and the Kuja Pirates (and potentially another group of allies tied to the Straw Hats) take on the Blackbeard Pirates. This war ends with the defeat of Blackbeard. This battle makes headlines, with the name "Captain Coby" being the one that makes the newspaper headlines alongside Luffy's.
> The conclusion of this war also sets up Luffy's journey to Laugh Tale.
But Wait, There's More...
So you'll notice that within that sequence of events, Luffy does not become Pirate King. In fact, he hasn't even been to Laugh Tale yet. The story I need to tell for my theory is complete, but that doesn't mean I've not thought about what happens further ahead. There's three very large players that have not been mentioned yet - Shanks, the Revolutionaries, and the Marines proper (i.e., not SWORD). Here's a summary of the events that I think will happen.
> Shanks and Luffy meet on Laugh Tale. Before Luffy is able to learn the true history, he reunites with Shanks. Based on the way Shanks' role in the series has developed (he seems more like a "guardian" than a direct threat to Luffy), it's entirely possible that they don't fight each other, though at the very least I expect a clash of Conqueror's Haki.
> News spreads that Luffy has reached Laugh Tale. His knowledge of the "True History", combined with the sheer number of allies he's picked up over the course of his adventures make him public enemy #1 to the World Government. It's at this point where Luffy's bounty surpasses that of Whitebeard and Roger, and he's considered the "Pirate King" (although this is totally not important, my prediction is that it will be 5.6 Billion berries). However, knowing what he now knows, Luffy decides that to achieve whatever Joy Boy wanted to achieve all those years ago requires the dismantling of the current world order - this may well be the same conclusion that the Roger Pirates reached, but were twenty years (or rather 25) too early to act upon.
> The Straw Hats, Red Hair Pirates, Straw Hat Alliance (which now includes far more than the seven crews Luffy recruited in Dressrosa) and the Revolutionary Army take on the World Government and win. Im may also fall as a result of this war.
> In the aftermath of this war, a group of leaders (Shanks, Dragon, the Gorosei, the kings/queens invited to Reverie) call a meeting to discuss the new world order. The Celestial Dragons are "dethroned" (this may be an area where Sabo and Vivi play a key part), and the true history becomes public knowledge. As a consequence of this new knowledge, piracy is no longer outlawed (extended thoughts on this can be found here). However, this group acknowledge that this free new era needs the presence of just Marines more than any other era in history - as the actions of individuals cannot be stopped. In this time, decisions are made on who will be the top brass within the Marines. This discussion concludes with Coby being promoted to Admiral - his exploits with Boa Hancock and in the war against Blackbeard, combined with his show of bravery at Marineford being the factors that clinch his promotion. Smoker may also become Fleet Admiral. Who knows.
> The seas are free, Luffy has achieved his dream YAY THE END.
This is very long I know, and it probably goes quite off topic in places but it's such an interesting idea to explore that I simply had to. Let me know what you think!