The inmates aren't imprisoned solely because they possess talent or because they used them to commit crimes.In the future, Japan may have moved into an Orwellian level of control to prevent the Tragedy from ever happening again.
They take those who society fears because they possess talent and bring them to the school with the intent of training them to use that talent to help mankind ala the Xavier Institute from X-Men. We know we are dealing with prison, but since the previous theory that the students are SHSL Criminals was jossed, the truth might be that the group is stuck there, because they used their talents to commit crimes.
For example: Tsumugi might have disguised herself to commit an assault, Miu might make killing machines, Himiko for thievery using illusionism, Maki might be a serial killer that uses kids as victim or bait and so on.
Following the line, maybe the characters are alternate timeline versions of the Student Council, since the Tragedy never happened, they wouldnt have killed each other, and the mastermind believes that is the way to restart the Tragedy on that timeline.
Plus, I believe the mastermind would be that timeline's Mukuro who wants to create a Junko to lead the world into despair.
As the title implies, the three characters we have seen so far are not in fact the player character.
There isn't much evidence to back this u except that one of Robokoto's argument's looks like it is getting destroyed in a trailer, and that including him there is a second character with an ahoge.
The creators did cite Given that they were from Class 74, it's not outside the realm of possibility that the two had a child before the final killing game.
The maid girl also appears to have gray hair, and if she's an Ultimate Maid she would have a similar talent to Chisa's.
We've already had an ultimate Biker Gang Leader and an Ultimate Yakuza.