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There are some problems here.
For one, all the Phantom Thieves are going to be base. They'd be stupid not to include Akechi since he's such a popular character. Kasumi is more questionable given we don't know when this game was developed, but given how Koei were able to incorporate Celica and Lyn into Fire Emblem Warriors I'm pretty sure she'll be in as well. If P5 gets DLC characters the only one I could think of would be the Twins unless they want to incorporate the Social Links. If they do, the sky's the limit there really. Regardless, Persona 5 is the focus.
I'd say the P4 team is more likely to be base than the P3 team, whereas P3 seems more likely to be DLC, but that's just me.
You say that, but remember that 2 of Awakening's most popular characters, Tharja and Owain, were DLC for Fire Emblem Warriors and Akechi was DLC for Persona 5 Dancing in Star Light with only a single song to his name. Akechi, as someone that isn't technically a Phantom Thief, isn't a lock and would be perfect fodder for a DLC character, especially if the story of Persona 5 Scramble is set after the events of Persona 5 akin to how Persona 4 Arena took place after Persona 4 Golden.
Again, another comparison to Persona 4 Arena, the focus of that game, and its sequel, was Persona 4, but characters from Persona 3 still appeared in both games, I could see them incorporating some characters from Persona 4 and Persona 3 in Persona 5 Scramble as well, even if the cast is primarily Persona 5 based.
I'm betting Social Links will be included in a similar way to how they incorporated Support Conversations in Fire Emblem Warriors.
I do think that Persona 5 Scramble will have more characters then Hyrule Warriors did..or at least the same amount as Hyrule Warriors + DLC + Legends + Legends DLC did by the end of it..so we probably will have all of the Phantom Thieves (Kasumi included), Akechi, and the Twins, and then probably some of the more popular characters from Persona 4 and Persona 3 such as Yu, Yukiko, Elizabeth, and Aigis as just a sample.