Obviously, no one knows for sure.
As time goes by, each player will find his identity at the club. There is even a possibility that he could become captain. However if I could choose, of course I would prefer Valverde or Bellingham to be captain because these two players are clearly much more mature and have leadership qualities on the field. Mbappe is better off focusing as a goal hungry striker.
As a leader, I would prefer to defeat Mbappe by zero votes. The qualities needed to be a leader are absent in Mbappe. If you watch Mbappe play and follow his game, you will see selfishness in him. He is even a goal-hungry player, he prefers to score goals himself instead of assisting. Sometimes there are conflicts with teammates even over penalty kicks. I would never choose such a player as a leader. To be a leader, you have to be more focused on the interests of the team than your own interests on the field of play and develop good chemistry with the players, which is the opposite of Mbappe's characteristics.