If Zetterberg were ever traded, I'm confident the team would go with 3 As to round-out the year. As far as the players are concerned, regardless of his play, all I've ever heard from the mouth of the horse(s) is that Kronwall is the voice in the locker-room, the most reputable player, the athlete you'd most want to emulate on the team, etc. That always surprised me but if he were around without Z, he'd be our man, whether wearing the stitched C or not. H'ed at least be the acting C.
As far as captains of the future go, I think everything about the organization's opinion of him from draft day, their promotion of him to the roster and as a PR player, and his role on the ice this year says that Larkin gets the next C.