If you want Xabi Alonso to prove himself for one more season in the Bundesliga, I think it will be difficult for Xabi Alonso to do that because Bayern Munich will not want any other clubs in the Bundesliga to beat them for the Bundesliga title next season. Secondly, Bayern Leverkusen will sell some key players, which will cause their performance to drop, and it will cause Xabi Alonso not to challenge for the Bundesliga title next season.
I agree with you that next season will not be this favourable for Bayern Leverkusen and I have already observed that on my previous post because Bayern Munich has been the most favoured club that has been winning the Bundesliga but after watching there performance this season and seeing how Bayern Leverkusen has taking advantage of the opportunity to dominate the league they will make sure it never happens again next season, so even if Bayern Leverkusen was not able to sell any of there players before next season but I doubt if there performance will still be good as this season, although I no that predicting what will happen next season will seem a bit difficult but with the way the season is going there is every possibility of Bayern Munich taking back the trophy next season.
Talking about Leverkusen for next season they might be will different because i believe some of players potentially will leave this club because the players such as Jeremie Frimpong, Florian Wirtz and the latest is Victor Boniface is into radar of big clubs so most likely those players will be leave or at least one of them will look for new club moreover to their manager too Xabi Alonso who rumoured will leave Leverkusen at the end of this season so if Leverkusen can able to gets Bundesliga title then i think it will be very hard for them to defend their title because they will lost some players and also lost their manager
And if this really happend which mean Xabi Alonso and some players leave this summer then i don't dare to putting Leverkusen as candidate champion for next season because clearly they will be different and for Bayern Munich although this season is not run well for them but Bayern Munich is strong club and admitting or not they are as the most consistent club compared to other clubs in Bundesliga because for every season they always be compete to gets Bundesliga title and for next season i am sure they will be more stronger than now with new manager