Do RPGs which introduce a new player character for expansion even exist? I only know of Baldur's Gate II: Throne of Bhaal and Dragon Age: Origins - Awakening, and even there it's optional thing, you can continue playing with your original character. So, I think V is a pretty safe bet.To introduce a new player character, it would certainely please a lot of players.
Better question is when the expansions will take place: pre-heist, post-heist/pre-ending, post-ending. I doubt the first expansion will be post-ending. Second - maybe.