Knowing that, in the Shanghai event Q&A:
Asagiri said Tanizaki's backstory is looking very interesting right now;
Asagiri said Verlaine's "storm" is only for Verlaine to know, but that his next appearance will be coming up.
The Tanizaki "siblings" have already had their charade and backstory teased in the manga quite recently;
Where Jun'ichirou goes, Naomi follows if she has any say in it;
Naomi is extremely capable and picks up on (dubious) skills easily;
The Armed Detective Agency is technically still indebted to the Port Mafia and would need to transfer over one of their members (should the deal hold);
Jun'ichirou has been described as strangely fit for assassination by a Port Mafia member before, Hirotsu;
Jun'ichirou has been described as the closest to the dark among the Armed Detectives;
Jun'ichirou himself has suggested he join the Port Mafia in Yosano's stead and;
Verlaine is in charge of assassin training and has done so with Gin and Kyouka.
I propose the theory (and just general concept) that Verlaine, with his mysterious storm that will not be revealed and therefore will not happen, is staying in his depression basement for his manga debut, BUT ☝️ it's because his role is to train the new ADA transfer for assassination.
Jun'ichirou has been teased for the capacity for evil and assassination several times in the manga, most notably in interactions with the Port Mafia. His and Naomi's backstory are becoming relevant, so they need to be close to the center of the action. We have a loose plot thread with the ADA->PM transfer. One or both could step into that role and take center stage.
PLUS, Asagiri loves rhymes in BSD's storytelling. We have a pair of "siblings", the boy an ability user, the girl nothing special but determined to follow her "brother", and they end up joining the Port Mafia together after the "brother" gets pulled in. You've heard this story before. What happens next? The girl becomes an assassin to keep up, and is trained by no other than the king of assassins, Verlaine.
So this would not only be a great time to explore the Tanizakis' teased backstory, but also the perfect opportunity to put in some Akutagawa siblings too. And since we're talking sibling relationships, Verlaine can participate with his own "brother" too!
In conclusion, it all fits seamlessly together a little too perfectly which means I'm shooting higher than reality will turn out to be but I will believe nonetheless. Also I need Naomi to be smart and weird out everyone with her usual roleplay bit. You wanted a transfer? You get the weird girl. Deal with it.