Bayer has a good squad, maybe Alonso will try to improve some positions, but he shouldn’t demand big finances for transfers. The next season will be a new test for Alonso, whether he can repeat the success in the Bundesliga, how Bayer will play in the Champions League, these are all very important questions. If Alonso had left for Bayern and Bayer's results had worsened, everyone would have said that the reason was obvious, but if he decided to stay, then he would have to work even harder, staying at the top is always more difficult.
Xabi Alonso doesn't even needs too many transfer by next season, a few transfer if necessary is all he will need because the build up of hi team is still very sound enough thst if they stay consistent with some form of improve, they are definitely going too end next season with another trophy, some how it may look luck for them that Bayern Munich may experience some instability for a while if tuchel leaves and that will be some form of advantage for them.
Leverkusen doesn't needs so much adjustments in their form and strategies of which I'm sure Xabi is aware and may not want to aswell tamper so much with what he has on ground, rather a strategy to make up for injuries and other reason that could cause a player to be absent should be made, probably seeing how well he can strengthen his reserve team just like Madrid does have so their strength will be such that they are most definitely not going to have reason to be scared even if they have a key player who's injured, one thing about the Xabi squad is their abilities of not depending on any player in particular which is definitely helping them a whole lot.
He can further extend this non dependency to his reserve squad so in cases where a switch will be required it wouldn't be challenging as the blend will be perfect and they will always maintain a perfect rhythm at all times, this will also contribute to their strength a d their chances of daring to repeat the same performance they had this season by next season even in a better way probably this time it will be challenging but it's still achievable.