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.
Bellingham may be a good player and he is quite reliable but I don't think he would be suitable as captain but we don't know for the future. Maybe things will start to look up when all these players have played more time for Real Madrid. Later you will definitely see who is suitable for it. And I think Real Madrid always needs a captain who is relatively older than the other players. And of course with good quality of course. And he must be a figure who is respected by all the players in the squad.
Everyone would rather see Bellingham captain Real Madrid than Mbappe. The English star is a very important member of Real Madrid's regular XI and the player has a very poor injury record, so Real Madrid will want to captain someone who performs well as well as being a regular full-time player for the team. I think all things considered, Bellingham would be the best choice to captain Real Madrid. When this player is made the captain of Real Madrid, the responsibilities of this player will increase which will result in this player performing better.
Best of luck to Bellingham.