I've narrowed it down based on characteristics and likelihood to start most games, and for me there are four main candidates:
Ederson - Calm under pressure and an ever present in the team due to his unique playing style.
Laporte - Seems like the ultimate professional. Essential to our way of playing hence why he starts virtually every game.
Bernardo - Leads by example with his work rate. Pep has said it's Bernardo plus 10 other players and won't leave whilst he's manager.
De Bruyne - You can tell he's incredibly driven. He's at the centre of everything and I think he understands Pep's philosophy better than anyone.
I suppose there's an argument that who the captain is doesn't really matter and it's more about the team but those would be my choices.