Real Madrid already signed Alaba, he can be positioned as CB, replaced Ramos. But it is good if there will be another top CB. Since Real Madrid played in UCL, La Liga, and Copa del Rey, they need more strong defenders, especially for CB. Nacho isn't young anymore, while Militão showed not really good performance in the last season.
David Alaba is a good player and he can play in many positions. It is a smart transfer of Real Madrid when they find another good and younger player with free transfer fee to replace Ramos. However, one thing I am worrying about Alaba, about his leadership. I don't know he can play well as a leader of Real Madrid defense system.
Leonardo Bonucci is a good defender, we all know it but he failed to be a leader of AC Milan defense system. Some players can not be good leaders in a system. They only play well when they are part of a system.