Given the impending return and a lack of theories to be instantly disproven once it's back, I figured I'd finally try to write this all down: I believe Tubeppa's Guardian Spirit Beast will spread Nen on the boat with a power that's already been used. The first part of this theory tries to explain past events leading up to chapter 400's ending scene, which is what I'm more sure of, while the second part is speculative.
Longhi, Tubeppa's bodyguard, is who kicked off this theory. To briefly summarize the situation going on in Kurapika's room, the unknown Silent Majority assassin has been killing people there. Benjamin's guard, Furykov, determines there's a hidden Nen user taking Kurapika's class and that they're the assassin. At the end of chapter 400, Longhi revealed that they could already use Nen, with the implication being that they're the hidden Nen user and therefore Silent Majority. However, given what's been shown in the story up to now, I think Longhi is neither Silent Majority nor a hidden Nen user.
To begin with, Silent Majority's actions run counter to Tubeppa's goals. Tubeppa wanted to use the classes as a chance to recruit Kurapika, ordering Maor and Longhi to signal their intent at an alliance during it. The killings, however, disincentivized participation, and Maor himself was among those ready to have Kurapika arrested over them. Belerainte's interruption is conflated with the idea that every SM candidate isn't hurting Kurapika's standing, whether intentionally or not. Longhi causing this scenario then allowing Maor to get the classes cancelled is absolutely not what Tubeppa would want.
Chapter 400's ending doesn't have to mean Longhi is Silent Majority, so we could step back and conclude that they're a non-hostile Nen user. Even then, it's clear that Tubeppa wasn't prepared for a battle of Nen, largely reflected by how desperately she wants Kurapika. When feeling cornered, her only thoughts are regarding how the newly awakened Maor can defend against Nen and how Kurapika is her saving grace. If Longhi was invested in Tubeppa's wellbeing, wouldn't the obvious move be to teach it to Tubeppa and the rest of the guards?
The handwave for this is that Longhi simply isn't loyal to Tubeppa. It's a good answer, but narration solidifies the loyalty of guards towards the eldest five princes. We do have Theta and Salkov as disloyal guards for the 4th prince, but they're the exception that proves the rule. Tserriednich is comically evil, they only took action against him once the prospect of him learning Nen came up, and Theta still can't help but admire him.
Could Longhi be disloyal despite all of this? Does Tubeppa secretly know more about Nen than what's suggested? Is Longhi withholding information for Tubeppa's benefit? Sure, these are all possibilities, but Longhi not knowing Nen seems to me like the simplest answer. The question then becomes why Longhi can use Nen when speaking to Kurapika. The answer to that lies with Maor.
Maor is Tubeppa's Captain of the Guards. In my opinion, the most significant spotlight on his character is when he stresses that what you make of information matters more than simply getting it. Togashi is big on foreshadowing via early character sentiments, and an opportunity for this to pay off already emerged during the Nen classes.
Once Kurapika begins awakening his students, it's done individually and we only see the process piecemeal. The end result is first shown with Ladiolus, and the tail-end of the conversation is shown via Yuri. It's also likely that Kurapika dowsed them during this one-on-one process, though we aren't shown that. Notably, the student whose POV covers the bulk of the information we get, including ways to awaken Nen early, is Maor.
Armed with knowledge on how to baptize others, and given Nen himself, Maor returns to Tubeppa's room. It's then that he would be back in the vicinity of Tubeppa's GSB, the frog. Up to this point, it has taken after Tubeppa's personality and decided to not show itself. But if Maor is the first Nen user among Tubeppa's guards, this would present a new opportunity. Many GSBs are aware enough to know who can or can't see them. Tubeppa's GSB would have little reason to show itself when it knows only enemies can observe it. GSBs also tend to appear when their host shows strong emotions, such as Cammy's will for people to do what she wants manifesting her manipulator GSB. When we cut away from Tubeppa, her last thought is (as shown previously) her need for Kurapika - her need for a partner. She strongly desires a partner and her GSB now has a Nen user who can be its partner; this is the perfect time for it to stop hiding.
Tubeppa's GSB is a "collaborative" type - the same term Kurapika uses when speaking to Longhi at the end of chapter 400. It also requires a "research partner", who in this theory is originally Maor. The frog can produce different drugs within its body. I imagine it's made clear to the prospective partner what it can do when the mechanical-looking beast reveals a lab within itself, speaks to them, or something similar. While making different drugs is neat, Zhang puts it best: it wouldn't be a Nen ability without some extraordinary power! A GSB that has to jump through hoops to make drugs via Transmutation is going to make supernatural ones with all sorts of potential effects.
It's probably obvious where I'm going. Maor learns about how to awaken people to Nen, and Tubeppa's GSB approaches him. Maor learns that he can make concoctions with the beast, and determines that these concoctions could awaken people to Nen as a byproduct of their use. Longhi, who has also been training to contain their aura shroud, takes the stimulant as a test. The experiment is a success, and Longhi is now fully awakened to Nen.
Still wanting Kurapika's mind by her side, Tubeppa believes she can use this tool as leverage for an alliance. This explains why we go from what I read as a slow burn recruitment tactic to Kurapika readily accepting a deal with someone already capable of using Nen with no apparent suspicions (likely post-dowsing). Longhi demonstrated the potency of the GSB's drugs in order to entice Kurapika into an alliance. So Longhi isn't Silent Majority, but still something that can propel the story forwards - the promise of a stalemate.
From here on my guesses become less grounded in what already happened, but we're starting off with reasonable stuff. The most natural consequence I see is Kurapika's stalemate initially being fortified. Now that general trust has been earned, the GSB can be used to speed up or bypass the lessons altogether. Kurapika wants this to happen to limit the actions of the older princes. This could pose a problem to Tubeppa, but her true targets are the eldest three. While she is looking to actively kill one through three, it seems she's only willing to totally write off Kacho and younger as non-threats - she just doesn't expect princes six through nine to participate. At this point, that means Tubeppa would most likely supply her drugs to the guards of Fugetsu and Marayam. Given her alliance with Tserriednich, she'd likely be willing to help him to some degree as well.
Among those who wouldn't get supplied by Tubeppa, Benjamin and Camilla already have Nen-wielding guards. Zhang, Tyson, and Halkenburg have Guardian Spirit Beasts seemingly built to provide proxy Nen powers, so they're likely able to match this force regardless. On top of all of this, Kurapika would presumably be working with Tubeppa to construct drugs that serve means besides rapidly developing Nen.
I don't imagine this would go over so smoothly. For example, Zhang is aware of Kurapika's importance and how Tubeppa has made moves on him already. Even if they do spread all this power around, Bill's fears could come to fruition. Tubeppa could become such a high priority target due to this that she guarantees herself a quick death. And one prince to especially consider is the only one in my hypothetical who wouldn't have the drugs or a Guardian Spirit Beast's power to make up for it: Luzurus.
To preface, I'm not inclined to believe any idea that Luzurus is a secret plotter. The first thing we hear reveals that he's not great with tactics as he let slip the possibility of assassination to his men too soon. Some take his GSB's power to imply something deceptive about him. I see it more as a play on the effects of hard drug use and Luzurus's thoughts on going too far with them. But with that being said, doesn't the premier drug user being left out of a large drug exchange feel a bit odd?
As established, Tubeppa considers Luzurus to be without ambition - she's overlooking him. For as of yet unknown reasons, the only real order regarding the Succession War Luzurus has given was to track Tubeppa. Despite Luzurus getting one over on Tubeppa feeling like it's in the cards, he leans towards inaction. His GSB can act to pick up the slack here, in this case by taking advantage of a transfer of goods. Something like a conjured vial that seemingly fell on the floor would be an easy way to seize control of someone moving the supplies, at which point they're manipulated into delivering the goods to Luzurus. While he might not know what to do with all of this, Luzurus has his mafia network to fall back on. He can go to Brocco for guidance, like Zhang did with Onior, and Brocco would fall back on the Cha-R.
This is many layers into guesswork, but I have enough reasons to suspect a spread of Nen within the Cha-R. First, balance is constantly brought up by the mafia, and is a big theme of the whole arc. The balance the mafia refers to is in many ways hypocritical, and they're aware of this. Power of the individual, the power of Nen, and preserving balance has been a consistent element on the boat. Sun-bin brings that up in reference to what makes the Phantom Troupe dangerous. While mortal enemies to the public eye, the three mafias still use exchanges to maintain balance. The Heil-Ly are like a lower tier equivalent of the Nen classes as they'll outpower both Xi-Yu and Cha-R once they have enough high-level users. If Ken'i ends up with some of the concoction and ascertains its purpose, empowering his own mafia seems more than needed to suppress the Heil-Ly as means of a Nen arms race.
Secondly, there is the chance for shifting alliances, such as the three mafia families entering a true three-way battle. The Heil-Ly uprising has put the two families on the same side, but there's still an air of tension around the whole thing. The Troupe can also become a major point of contention. While Hinrigh is a Spider stan who has become greatly endeared to Nobunaga, Ken'i is actively planning how to kill them off. In general, Hinrigh has been a protagonistic POV while Ken'i is more mysterious and antagonistic. Add to this division how many factions would find it convenient to eliminate the mafia, and you have a scenario where the Cha-R spread Nen as a means of self-preservation while simultaneously distancing themselves from the other families to avoid the ire of other institutions.
Third, the Cha-R have members that would be prime candidates. From the group shot we have the emphasis placed on three characters. Sun-bin is a confirmed Nen user, and I'm assuming that the middle character is one as well. If so, they and Ken'i would mirror Xi-Yu's trio. The lieutenant (presumed rank as he mirrors Connelly) relying on Sun-bin for the Troupe's aura analysis, it being younger members that use Nen, and Ken'i warning all of them about Nen users strengthens this likelihood that the others aren't Nen users themselves. I'm not totally sold on this, as I wouldn't rule out this being a strike team of Nen users, with potentially only the old man not being one. Either way, the drugs can enhance what's already there, or we could see other related characters get their hands on the drug.
One higher-ranking Cha-R we can safely assume doesn't have Nen is Tsudonke. His autograph board hunt should have some payoff. The Hisoka search seems to be moot. It might be a roundabout way to establish the delivery system, but Togashi has bothered giving him an extremely specific, consistent, emotional investment. If anybody in the Cha-R needs Nen payoff, it's him. This could also cause an interesting dynamic where their strike force develops abilities with the aim of countering Spiders while Tsudonke has one to potentially support them.
Also worth mentioning are Tserriednich's friends. They're working alongside the mafia and curious about wielding Nen themselves. Tserriednich receives some of the shipment as well in this scenario, so if Gipper maintains contact, it could actually be their group who informs Ken'i about what the drugs do. I agree with the theory that Morena will capture one of them, specifically Momolly. It's an ironic introduction, which Togashi is fond of, if Momolly gets an unwanted kiss. I'm also expecting most/all of the three unnamed friends to die, leaving Borksen and Otocin to take their chances with fast access to Nen. Borksen's friendship with Theta could influence her ability towards restricting Tserriednich while Otocin's ability could play into wanting to "save" him.
Just as the spread on upper levels would make Tubeppa a high priority target, the Heil-Ly could associate this with Luzurus and make him their own priority target. Tying special drugs to Luzurus's own ability is an easy assumption. We've had emphasis placed on the Cha-R secret passage, with Phinks mentioning murdering Brocco as a potential reason for reaching it. Access to Brocco's room could also double as a means of ambushing Luzurus, and we may actually see a Heil-Ly gain 50 levels for their effort.
It doesn't have to specifically play out this way. The main idea is that this won't just be spreading Nen, but escalating tensions and forcing people to take actions across tiers. Tubeppa and Luzurus may die for it, but not before they've armed dozens of new Nen users and set things up for an explosive finale.
TL;DR - Longhi wasn't a Nen user before, and was actually awakened by Tubeppa's GSB after Maor learned Nen. Kurapika will ally with Tubeppa to make use of her beast, leading to a spread of the Nen-stimulating drug. This will result in Kurapika's desired stalemate but could become its own problem when the stalemate breaks. There's also the potential that Luzurus ends up with some of the drug via his GSB. He could then look to the Cha-R for advice, leading to the drug awakening people in lower levels. This would mean that one little frog could potentially accelerate both the upper and lower tiers, priming the arc for its climax.
Thanks for reading! It's hard to write something totally encompassing since there are many intersecting parts in this arc, so I tried to leave out stuff that felt too tangential. Feel free to share any questions, points for/against the idea, or other things you think would happen if this ended up being true.
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