First off, this is probably as optimistic of a theory I will ever write, which should make sense because both the manga and anime are peaking in terms of excitement. If you are manga only, you HAVE to be at least skimming the most recent anime episodes. I probably haven't been this excited about the manga since I first caught up to what was current content at the time. Visual references will probably minimal to nonexistant since most of the building blocks should be major plot points. Also, I want to completely skip over the potential details of the power. If that's what you're interested please post a theory thread or give feedback on my earlier theory here.
So this actually might take a bit of theorizing despite the simplicity...which makes it kinda fun. Luffy getting his awakening/"Gear Fifth" seems like a perfect fit for the only chapter in all of One Piece to end with triple zeros. However, SOOOOOOOOOOOOOOOOOOOOO much stuff has to happen before this. We have TONS of action right now. This alone could go all the way to chapter 1000...only if Oda wanted to have a lame event for the monumental chapter though (referencing the minor fighting FYI).
My key evidence is what Kaidou mentioned on how taking down the leader results in the alliance falling apart. This has come up in multiple discussions, but this is how Oda will rationally explain the collapse of the Kaidou and Big Mom alliance (everyone has their own agendas and will go their separate ways once the tyrants are not there to force them to all stay together => there's more visual friction among the Charlotte kids so that's guaranteed to happen). This is also the theme for how ROCKS fell apart. It also was foreshadowed when Shiryu mentioned the Blackbeard execs would all go their separate ways if Blackbeard failed to steal the Gura Gura ability (damn, only realized this part now!). Either way, this is the common theme that will semi-apply to Luffy's alliance.
Luffy's defeat is going to force everyone to flee (and potentially every person for themselves). Luffy is the unifying factor. He is why everyone has blind faith in him and his dream (see Kinemon verbally standing up to Kaidou a few chapters ago). Luffy losing, and especially if it's at the hands of Big Mom rather than Kaidou, is going to crush everyone's spirit, and rightfully so. This is befitting of the "tragedy" aspect of Wano Kuni's third act. Everything has to be befitting of Yonkou level (including the massive build up to this arc).
There has to be some sort of twist to save at least the key protagonists (the samurai are sent to Udon, so no worries on executions unless it's a major ally). For simplicity, this has to be whoever will teach Luffy his awakening. Since the regulars already know my thoughts on this, I'm skipping over. For new readers, please feel free to tag or quote and I can expand as desired.
Next, there has to be training. This isn't that big a deal based on Oda's most recent skill of purposefully creating anime-only content. Luffy struggling through training is one example of this. More importantly, this applies to all of these key steps. Gonna go on a big picture tangent, but the best use of a long serialized manga and anime combo is for the manga to be the template and major outline for the anime. Purposefully having anime only content that is relatively huge canon content, this drives anime viewership. This looks like it's happening and hopefully consistently.
Luffy sucks at haki, so I'm kinda glad we didn't have to see Rayleigh beating Luffy up for him to slowly get it. There's probably still a short time skip since there is no way for Luffy to consistently activate his awakening in just a few days. Either way, the training only has to be part of a chapter, which means the second half of chapter 999. That leaves 10.5 chapters (989 through the first half of 999) to get through the two other key aspects. This seems readily achievable based on Oda's dramatically accelerated pacing.
Splitting the continued build up (teasers of action), Luffy's defeat, and the escape equally leaves about 3 and 1/3 chapters for each. So IF Oda's relatively break-neck pacing can continue, in conjunction with purposefully including anime-only content to fill in the gaps, we can speed ahead to Luffy truly becoming "Yonkou-level".
We should be able to universally agree on the main cliffhangers to end chapters based on the above key points, so we can track the pacing if it's on track or not. Even though it is a tight squeeze, it is definitely possible. Oda has NEVER progressed his plot at such a fast pace (in my close to 15 years now). This is a huge part of my excitement on the potential content. Luffy becoming strong enough to be world-changing (and allowing the transition back to character-driven plot development), is something most fans want/seek unconsciously. I cannot think of something that fits more perfectly for the significance of chapter 1000.