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.
The burden on Bellingham's hips is too great if Real Madrid has to trust him to be captain, of course there will be a lot of burdens that must be done as a leader on the field, not just playing or whatever, but it is not easy for him to do that at an age that is still very young, maybe it would be much better to provide a player who is more prepared than him.