In fact, the challenge for Xabi Alonso is next season, because maintaining his performance will be more difficult to do, and if he fails to maintain Leverkusen's performance this season, then he will be considered a coach who just had great luck this season by being able to bring Leverkusen to the championship. and they even have the chance to win the treble.
Yes, it is always a big challenge to maintain good performance. Leverkusen performed too perfectly this season, they may end up this season with no defeat at all. As far as I know, there is no modern coach who can do this. Xabi Alonso simply did an extra ordinary job for Leverkusen, no one predicts this to happen with Leverkusen.
Well, treble winners looks very possible for Leverkusen if they can maintain their performance until the end of the season.
With the squad he currently has he can show an extraordinary game and next season he can bring in new ammunition to complement the squad he has now. Next season they will be in the Champions League, the competition will be more difficult there and he has to prove that he can compete.
Even if the current squad looks strong enough for the next season, I think Xabi must improve the quality of the squad. I think Leverkusen has no top striker, most of them are just average strikers. Only Victor Boniface who deserves to be the striker in Leverkusen. Sadly, this player got a long injury this season, Xabi must have an alternative striker who has the same quality. Patrik Schick and Borja Iglesias have too far quality, they don't deserve to keep on the squad. If Leverkusen wants to go far in UCL for the next season, Leverkusen is better to sign a top striker in the next transfer window.