Yes, if it were me, I would also choose Mbappe, Mbappe is probably more mature and has more leadership qualities. Vinicius is overconfident with his talent, his personality, and his attitude of course that is what directly affected the recent Ballon d'Or when he was not awarded. And this opposing competition between two top strikers is the reason for Madrid's failure in attack.
First of all, I believe Modric is still the leader at the moment, as long as he is there, he will be the captain and the leader of that team, but after he leaves, my suggestion would be Courtois, that dude is the best goalkeeper in the world, and won so many cups for them with his amazing saves, and he has the age for it as well.
I believe that would actually work well for them, and they could give him the rights to be the leader. Vini and Mbappe feels wrong. Also, Ancelotti may leave, that much is true, but I do not think Xabi will have an easy job there, I do not expect Xabi to accept that offer, come in, and win every game. He will have hard time with such a huge club because he never managed a big one like that.