Well, I'm hoping/expecting one in particular, and it's the one that's a P4 spoiler so I'm always unsure about mentioning it in these threads outright.
There are two characters that seem like the obvious choices: the P4 spoiler character, or the P3 spoiler character. I always thought there would be at least one additional, story important character after Rise and Ken. Some had speculated that they think there'll be two additional characters because of the current P4AU character select screen that has four static slots at its extremities, but I think those also just might be purely aesthetic.
Either way, all of the most obvious choices have been chosen, so whoever it is will be pretty interesting to see.