Next season will be a real challenge for Leverkusen. They will play in the Champions League and that is a high level competition. Xabi must be able to rotate players and add variations to match tactics. I think club management will give Xabi a budget to find the players he wants. And for tactics I think if you play at a level like the Champions League you really need a variety of tactics because strong teams from other leagues will look for your weaknesses. And if they find your weaknesses then the teams in the domestic league will also find out your tactical weaknesses if you lack tactics.
Bayer Leverkuse will have to face with a very chaotic summer because not all of their star players will want to stay with the club. Some of them will want to leave and find other bigger clubs to satisfy their ambition. Leverkusen will receive many big offers from big clubs for star players and they will not be able to keep all of their star players for a next season. This is first big challenge for Leverkusen's board members to solve in the summer.
There is another big challenge, next season, playing in Champions League will be big difference than this season. It will force Leverkusen to focus on Champions League and they will have less attention on Bundesliga. Even they want to play well in Bundesliga, defend their title, it will not be easy because they don't have too deep player resource.
The next challenge is that Bayern Munich will be better prepared for Leverkusen. I'm sure this season they really didn't expect Leverkusen to be a problem for them. And they definitely won't let that happen again next season.
This is another big challenge for Leverkusen, simply because Bayern Munich will not stay quietly and do nothing. They will buy players, enhance their squad, find a new coach and challenge Leverkusen to win Bundesliga again.