1. Joker
2. Morgana
3. Ryuji
4. Ann
5. Yusuke
6. Makoto
7. Haru
The Phantom Thieves from the original game - they all have to be in here if this is a Persona 5 spin-off. (I feel like Futaba's Persona could potentially work in combat if needed, but I haven't played Persona 5 Royal yet, so I want to do that first before including her in something like this)
8. Akechi
9. Kasumi
I thought about adding Zenkichi, since I wanted to acknowledge Strikers, but by my understanding, he outranks the regular police, which could present a problem plotwise, so if need be, he could be assigned to an important case. By my understanding, Persona 5 Royal establishes, or at least hints, that Akechi survived his apparent death in Persona 5, though he presumably wouldn't be on the protagonists' side.
The reason why I left out Zenkichi is because, much like Persona 4 Arena, we'll be bringing back some older characters who have been aged up a few years. (I'm pretty sure that while Persona 3 and 4 have a clear amount of time pass between them, Persona 5's timeline is more vague, though I think it's been at least five years) Many of them are affiliated with the police, so Zenkichi having the ability to override them would limit their role in the plot.
Sophia could be playable (some justification could be included for her to make a physical appearance), and I figure she'd have good interactions with the casts of the other Persona games since they're familiar with Aigis and Labrys, but she has plenty of design overlap with Kasumi, and I'm not sure how much space I'll have since I'm over halfway through the roster at this point.
10. Yu Narukami
11. Chie Satonaka
12. Naoto Shirogane
Naoto seems like she'd want to investigate these "Phantom Thieves", especially as their ability to use Personas becomes clearer through her research. (between Persona 5 and Strikers, they haven't exactly been subtle, and they've gone from impacting one city to places all across Japan) Chie's part of the police, or at least in training to become an officer, and while they might not be able to get the whole Investigation Team together on such short notice, they'd presumably try to bring their leader (Yu) in to help, at least.
13.
Master Joker
You're probably saying "who?", and admittedly, I was only vaguely familiar with this guy and had to look him up on the Persona wiki. Still, one of the antagonists of Persona 2 went by the name "Joker", so if the Phantom Thieves are going to be framed for something that could lead to the Persona 4 cast getting involved, going with an antagonist who has the same pseudonym as the protagonist seems like it would help.
At the end of the game, Master Joker would be revealed to be a fragment of
Nyarlathotep, the main villain of Persona 2 - he gains his power by spreading rumours and having them turn into reality (which ties in pretty nicely with the themes of Personas 4 and 5), and Persona 2 ends on the note that he could return, so maybe a small fragment makes its way into the world and is trying to spread chaos and confusion. Nyarlathotep could be a boss-exclusive character, either using his Joker-Nyarlathotep form from Persona 2 (pictured on the left) or an unused form that got artwork. (pictured on the right)
As he's a fragment, he's weak enough (relatively speaking) that he can be destroyed through combat. Admittedly, I haven't played Persona 2, so I don't know how well this could fit, but I didn't want to go with "here's my OC; do not steal".
If I can fit two more characters...
14. Mitsuru
15. Aigis
Akihiko's absence seems easy enough to explain (he's training in the mountains so nobody can contact him), and the Shadow Operatives would almost certainly get involved with anything Persona-related - I'm surprised they haven't showed up in Tokyo already. Persona 3 is where the series took off, so I figure it should get something, even if it kind of feels like the barest minimum.
The plot of the game is that Joker (once again) and the Phantom Thieves are framed for a crime that they didn't commit - MURDER! Naoto and Mitsuru were aware of the Phantom Thieves, but allowed them to continue operating because outside of that time with Haru's father, nobody was dying as a result of their actions, so Naoto and the Shadow Operatives figured that they were on the side of good.
Naoto and the Shadow Operatives manage to narrow down the location of the Phantom Thieves to Tokyo, as that's where the vast majority of the changes of heart took place in the first year that the thieves were around. Now Tokyo becomes a battleground between Persona users - the eight Phantom Thieves on one side, the Shadow Operatives and Investigation Team (or what few members they could gather on such short notice, anyway) on the other, Akechi as a wildcard who can help or hinder the Phantom Thieves, and the mysterious Master Joker who's pulling the strings for his own purposes.