I think Bayer Leverkusen winning the league next season is under 70% this is because there is no news of any of their player who has left the club at the end of the season even though they have not added a new player yet in the team but the availability of them manager Xabi Alonso still means a lot to the team and it will give them an edge over their biggest rivals Bayern Munich because they have a new coach who will take some time to master the players before chosing the write tactics and formation that will be best for the team.
Bayer Leverkusen involvement in other competition especially the champions League will surely affect their performance in the league but since their opponent Bayern Munich and Stuttgart is also playing in the champions League I think they will still have the advantage of winning the league because they have a more balance team than Bayern Munich so far.
The Bundesliga will send five representatives to the Champions League next season, Leverkusen, Bayern Munchen, Stuttgart, Leipzig and Dortmund. Meanwhile, I predict that the competition for the Bundesliga trophy will involve these five teams, so those who can appear balanced in these two competitions will be very lucky. Leverkusen has the better chance of them, because Xabi Alonso decided to stay for next season. Meanwhile, the depth composition of Leverkusen squad doesn't seem to have changed much, so they have the opportunity to repeat the success story like last season.
Regarding Leverkusen performance in the Champions League, I assume their journey there will not be as consistent as in the Europa League last season, because there are many elite clubs who have better strengths. Even though Xabi Alonso will try to prioritize these two competitions, I think his main priority will still be the Bundesliga. Meanwhile, changing managers for Bayern Munchen and Dortmund should be a good opportunity for Leverkusen, because usually new managers need time to adapt. Personally, I would not be surprised if Leverkusen will win the Bundesliga trophy again next season.