So Shadow characters can't Burst, but they have a full-meter boost that sort of works like Masukukaja? Crazytown. It doesn't double the number of playable characters, but it adds a neat dimension. I'm okay with this.
And I would STRONGLY doubt Shinji being added as a playable character, for story reasons. My guesses for a potential new additions are either characters created for the game or possibly a P5 character, sort of like how Roy was added to SSBM as an advertisement for the then-upcoming FE6.