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.
Bellingham is actually suitable to be the team captain because he has a leadership character and can encourage the team when they are frustrated or whatever. But we all know that Real Madrid always has a tradition in having a team captain, they prioritize senior players, players who have served the club for a long time who become captains and so on down the line until whenever in Madrid. They are not another club where new players can become team captains.
So Real Madrid with the tradition of choosing a captain, of course the next captain is those who are senior and those whose deputies are below them. Like in the era of Hierro, Raul, Casillas, Ramos, Marcelo, Benzema, Nacho, and now Calvajar and after that Modrick, Lucas Vazquez. So if you look at the tradition, Bellingham and Mbappe still need a long time to become captains at Real Madrid.