I think at this point I'd be STRONGLY worried about Atlus if it wasn't Persona 5. There's no telling it is or isn't of course, but yikes if it isn't.
Even if I really want it, imagine how bad blowout would be if this was just for a new release of Persona 3?
If it's a significant product, despite it not being P5, why would you worry about the company itself? I could understand being worried about Persona 5, but not Atlus.
It'd be pretty crazy if this was for a Persona 3 re-release, but like I explained in a previous post, none of the teasers they've made for games in the past have been something "small" like that. Anytime it's been a re-release, even with Persona 4 Golden, they just outright announce it in Famitsu.
They don't tease things for nothing.
EDIT: I really hope they put in as much work for all of the other shadow variants for the characters as they did for Kanji, because that's really making me excited for the game.
Of course they did. Shadows are their own characters.
"Shadow Aigis has an intro where she crushes a black butterfly in her hand"
"Shadow Chie tramples the opponent’s head on victory"
It'll be like that for everyone. The ASW side producer has already assured people in an interview that they'll be fully unique. They'll have their own VA, intros, win poses, portraits and all that.