Leverkusen will certainly have great difficulty repeating the same achievements as before. Competition in the Bundesliga is expected to increase after Bayern Munich and Dortmund start the new season, as well as other matches in Europe. Leverkusen will play in the Champions League, but they will probably be thrown into the Europa League if they fail in the group stages. They will never be the same even if Xavi Alonso will try his best, but still have a good chance.
I never think Leverkusen can match their unbeaten record next season, but everyone can expect logical things about them next season. Xabi Alonso has a big responsibility to maintain his team's performance, this is the most difficult challenge I imagine for him next season.
Of course, repeating the same achievement as before is very difficult, especially maintaining an unbeaten record is very difficult to do. because of course other Bundesliga teams have also made improvements as you said, making them even stronger. However, in my personal opinion, Leverkusen's chances of retaining the Bundesliga title are very big, especially since Dortmund and Bayern Munich have changed coaches, of course with a new coach the players have to adapt again and this takes time.
Leverkusen this season has only suffered one defeat and that one defeat was very painful because they lost in the European final against Atalanta. However, looking at the format of the UCL league next season and the very tight competition in the UCL league, it is not impossible that Leverkusen will be relegated to the European league. If this scenario happens, I think Leverkusen must be even more motivated to win the Europa League title. Even though they didn't go down to the Europa League and continued in the UCL League, being able to reach the semifinals of the UCL League for Xabi was an extraordinary achievement because the atmosphere and teams in the UCL were very different from the Europa League. .