Considering the stuff we got in the MCC trailer and then in the bookend cinematics, thinking Arby would be a playable character seems foolish.
Speaking of playable characters, though, if the four-man teams are your coop buddies in full party situations, I wonder if that means they'll still have speaking roles. For a long time I always felt that silent protagonists during gameplay were the way to go because having a voice that "wasn't me" felt off, but either I've changed or games have gotten much better at it. I vastly preferred Halo 4's in-game dialogue with Cortana to getting ordered around like in previous games, and other games like Bioshock Infinite have really made me hate it when my character is a dumb mute these days. So would you as Kelly be responding to another player as John, et al?
One thing's for sureyou're not going to get casual players to care at all about the characters unless they've got a lot of time put into characterization, and really the best time to do that is during the actual gameplay itself.