>Sensei receives a call from Erika
>Tells him to come to Gehenna because there's chaos brewing
>Obligatory "must be a day ending in Y" -joke
>Erika explains it's actually serious and she doesn't even want to elaborate on phone out of wiretapping concerns
>Sensei arrives at Gehenna
>Turns out Hina and Makoto have both disappeared
>Erika discovered it by complete accident, Prefect Team and Pandemonium Society have managed to keep the situation under wraps otherwise
What happens next? I dunno. To make it proper Peak Archive would require setting up a twist that ends up being a double subversion of sorts. Maybe it's rumored that Hina and Makoto, despite being blatant rivals, are actually organizing some kind of grand conspiracy. But then it turns out they were both sent out to a mandatory vacation/bonding experience, and that's why neither the Prefect Team or Pandemonium Society alerted Sensei. I don't think this would carry an entire volume, but it would at least provide an opportunity for a Sensei speech about how Erika was acting in good faith, based on what information was available (assuming she gets blamed for calling Sensei and accidentally escalating the situation.)
If on the other hand you want a proper Volume that fleshes out Gehenna' internal politics, you could keep the "Hina and Makoto have disappeared or gone to hiding or conveniently on some kind of state visit" and the problem is some kind of political agitator loosely based on Karl Marx? I'm kind of spitballing at this point.
Another possible inspiration is the idea that Gehenna is based on West/East Germany, instead of (or at least in addition to) the WW2 vintage.
BTW... I've read a theory that the Thunder/Tyrant Emperor was actually based on Wilhelm II and not the Austrian Painter, but it was based on a "it came to me in a dream" -kind of assertion that the Gehenna's Emperor abdicated voluntarily instead of self-breaking her halo. Actually, I remember now: it was stated she left Kivotos. But it was left vague enough that it could've been an exile like Wilhelm II, regular graduation (whatever that actually means, considering characters like Shun and Kokuriko) or euphemism for something worse. Probably should address this one way or other. Either elaborate on it or establish Gehenna has an official policy: "First rule about Thunder Emperor is that we don't talk about Thunder Emperor."