I also think Xabi could have a successful career with Real Madrid, because despite a very difficult period at Leverkusen, they didn't suffer a single Bundesliga defeat in the season they won the championship. Even taking into account European competitions, their only defeat was against Atalanta in the final. Now, with a team like Real Madrid, and with the resources they possess, they can achieve even greater success. As long as they compete with a systematic and success-hungry squad like Leverkusen's, I believe Xabi will serve Real Madrid for many years.
Xabi Alonso is a smart guy and he will know that if he wants to be successful with the team, he will have to control all the star players and maybe send someone away because the team is too crowded with individual stars and each of them wants to get his way, which can't work. Xabi has to get that under control, otherwise I don't see any chance of great success in the CL.