Okay, so the canonically confirmed Cang Qiong peaks are:
Qiong Ding (leader peak, CEO peak, politician/diplomacy peak, admin peak, etc)
Qing Jing (scholar & artists peak, knowledge peak, strategic advisor peak, etc)
Wan Jian (sword guy peak, blacksmith peak, armory peak, etc)
An Ding (logistics peak, servants peak, peak of the cang qiong labor party, delivery guys & messengers peak, supplies & catering peak, etc)
Xian Shu (gender segregation peak: girl flavor)
Bai Zhan (shounen anime peak)
Qian Cao (healer peak, pharmacy peak, first responders peak, etc)
Ku Xing (gender segregation peak: boy flavor, ascetic peak)
So we only have three peaks that are entirely unnamed and unaccounted for. This has pretty good utility if you want to do a transmigration fic where Airplane and Shen Yuan are there as peak lords, but so are OG Shang Qinghua and Shen Qingqiu. You can just give them two of the three mystery peaks, and then there's only one peak remaining for an OC and kapow, you've got all twelve peak lords sorted.
But the question of course, is what should these peaks actually be? What should they specialize in?
Fandom has argued in favor of a Beast Peak, and I actually endorse this idea, although I think specializing in demonic beasts is more of a Bai Zhan and Qing Jing thing. But while we can suppose that between An Ding, Qian Cao, and (probably) Zui Xian the agricultural needs of the sect are being met (or met sufficiently for what they can also supplement through trade), there's no clear existing peak to outsource things like training spirit animals or keeping any livestock that the peak might require. And hey, if there is a Beast Peak, then them also having some expertise in demonic beasts would be interesting.
I think the Beast Peak would slot in most logically between Xian Shu and Bai Zhan.
For the lowest peaks, things get more interesting. While there are obvious roles such as talisman making, barriers, musical cultivation, etc, most of those things seem like they'd either be covered by one of the other peaks (i.e. Qing Jing and musical cultivation) or else would be strange things for the sect to acquire so late, literally after the peak that specializes entirely in brewing.
But, that actually can work out, if we assume that these peaks have taken as specialties things that were previously secondary or tertiary interests to other peaks. Perhaps even owing their origins to particularly capable disciples from the other, more highly ranked peaks who showed such prodigal skill or innovation in that area that they were allowed to establish new peaks focusing on it.
For my money, I'd go with a Barrier Peak, specializing in protective barriers, talismans, and spiritual cultivation that shot off from Qing Jing during some long-prior generation. This peak could also be responsible for guard duties in the sect, basically sending disciples to close off unsafe or prohibited areas, to manage things like access to the various branches of the Lingxi caves, sealing off dangerous items, and (probably) helping to maintain existing barriers, arrays, and other such systems throughout the sect.
I think this peak would be a decent fit for Airplane, as it would once again situate him pretty close to matters of daily sect operations, and put him in position to know a lot of the secrets and goings on beneath the surface of things. So a plausible explanation for his authorial knowledge and insights would simply be, Barrier Peak are the flies on the wall of a lot of high-level matters. If someone breaks into a restricted area, they know about it. If someone wants something hidden, sealed, or disguised, they know about it.
Bonus angst: this would probably mean that the Barrier Peak's head disciple assisted in sealing a young Yue Qi inside the Lingxi Caves with Xuan Su. If that's Airplane, well, that's twisting a knife a bit now isn't it?
Which just leaves peak no.12, which frankly could be any damn thing. After Booze Peak and Girl Peak, the field is wide open. Dance Battle Peak. Transit System Peak. Spiritually Infused Textiles Peak.
My personal favorite, though, is Sex Worker Peak. Not only because that is the most fanfic-y option, but also because it actually kind of makes sense.
The PIDWorld is just chock full of fuck-or-die tropes, which makes there are countless substances, ailments, curses, etc that can only be cured via sex. Not just for cultivators, but also for everyone else in the world. Like imagine you're an NPC magistrate or something just out there managing your district, having only the most tangential connection to the plot, and one day you're going for a walk and you trip and fall and manage to land right in a field full of sex pollen that cropped up like weeds overnight. Because that's just how this shit works, it doesn't wait for the protagonist to exist in order to activate, it's all got to be out there all the time in order to be there when the wife plot happens, and also for various experts to have accumulated all the mandatory exposition points about how it works.
But you're just some normal guy! You don't want to die of Horny, but the best way to clear this up is not just to have sex, but to have sex with a cultivator who is at least moderately good at using the exchange of spiritual energy to purge your body of the sex pollen poison.
Unless you're lucky enough to know someone, you're probably going to be in the market for professional help here, like even apart from all the other reasons people like to hire sex workers. This is a situation that probably happens fairly often and for which "hire someone to fix it and then move on with your life" is probably the ideal solution. As a bonus, a professional sex cultivation expert is probably also going to minimize your risks for unwanted side effect like STDs and pregnancy, too.
So, imagine we have Qian Cao peak struggling under the workload of all these requests for help with dual cultivation. The problem isn't prudery, but that this stuff is so commonplace it eats up time that could also be allocated to things like research and other medical emergencies. Plus, you have political leaders (kings, princes, emperors, etc) always demanding to be sent your "best" disciples to attend to them, when quite frankly their condition is something even an outer disciple could handle in less time than it would take them to travel out to their location, and these fuckers are not-infrequently liable to try and steal your people away into concubinage too.
One day then, much like with Qing Jing and the Barrier Peak, the Qian Cao peak lord of yore gets fucking fed up and is just like, this requires it's own department. Zhangmen-shijie we're starting a new peak. I'm not asking you I'm telling you. It's a medical peak entirely devoted to sex work. My best disciple at sex, who is in the running for Head Disciple status almost entirely because of this shit, is going to be the new Peak Lord. Any time some princess gets her vagina cursed and needs dick badly, the new peak are going to handle it, while I get to finally fucking finish my research into organ transplants.
And the sect leader of that era, knowing what was good for her, was like yes okay rubber stamp that we have twelve peaks now. Twelve's a good number we probably should have done this sooner anyway. What do you mean we don't have that much mountain? Eh, we'll haul some dirt in and make it happen.
Other Sex Worker Peak Thoughts:
Obviously, raising disciples from the age of ten upwards into this kind of work is controversial at best. Depending on tone, a fic author could either accept that grooming children for sex work was a historical practice and examine the fucked-up-ness of it all, or, we could go another direction and make a case that this is generally the peak which takes on older prospective disciples.
After all, dual cultivation is actually good at helping with setbacks and restoring a damaged cultivation base. You could argue for it being the ideal cultivation approach for latecomers. There could even be a precedent for adult disciples from the other peaks transferring to the Twelfth Peak/Sex Worker Peak if they show an aptitude for the work, and for disciples to temporarily join them as part of repairing or preventing damage from qi deviations.
This could also be a contributing factor to Shen Jiu being like, I have to not only be on Qing Jing Peak but also be the absolute boss of Qing Jing Peak with as few people able to gainsay me as possible, because he's terrified of being ordered to pimp himself out.
Not that he would be, though, because I imagine the sexpert cultivators are pretty well aware of how trauma works and who does or doesn't actually have the right temperament for their business, or what jobs within that business. It's their specialty, after all. If someone is going to have a panic attack and qi deviate over doing the job, that someone is not a good candidate for the job, or for these types of treatments overall.
Sometimes Twelfth Peak loses people on account of them falling in love with their clients or deciding to take some king up on his concubinage offers, but it happens less than one might think. After all, it's basically like working for the best brothel in existence. They have rigorous hygiene and healthcare standards, you get access to all the generalized medical care from Qian Cao, travel expenses are covered and you don't have to work out of your home if you don't want to, your food and housing is supplied by the sect, you're trained in cultivation and martial arts, with a shot at achieving immortality, and you don't even have to work every day because the jobs are contingent on what's being requested, not on you making rent money. In addition to physical cultivation, you can also make and sell tons of erotic art or "love tokens" and it will sell for a lot because of the social mystique of sexy cultivators. A pair of twelfth peak lord's panties probably goes for just as much as one of Shen Qingqiu's fancy calligraphy paintings. So unless you really want to live with some dude, switching over to depending on him for your upkeep doesn't seem all that appealing as a prospect.
Additional fun with this idea is that it would also potentially be an interesting peak lord role for either Airplane or Shen Yuan to end up in. Airplane would probably be like, well I guess this is karma for putting so much gratuitous smut in my stories, and then actually manage the hell out of the whole peak and enjoy himself by only taking on the jobs he actually cares to. Not a bad gig, especially compared to his previous grind. On the other hand, Shen Yuan's internal freak out and subsequent attempts to somehow be the Sex Peak Lord while not actually having any sex would be a potential comedy/suspense goldmine.