If they want to maintain their performance next season then Leverkusen should try to add depth to their squad because they will be playing in the Champions League. The decision to stay is much better than leaving too quickly to take charge of another team because Xabi Alonso needs to prove his quality as a coach. Playing in the Champions League will mean that Xabi needs several quality players because the matches there will be much different from the Bundesliga.
I think the management will try to find a solution to add depth to the squad next season. Because they cannot possibly rely on a number of existing players next season because they are worried that if they experience injury problems, their plans to compete in a higher caste will be hampered by these problems.
Talking about depth of a squad, I want to bring to your mind that the dept ofa team was at some point who they actually did groomed, those players who they will sought from out side to add to their depth were once like the players Leverkusen currently had before they got the quality they have gotten now and would look like an addition to the depth of a team so I think just going to sought players for depth isn't Leverkusens problem at this time.
If Xabi is staying back I think improvement and build up should be their target and they will have the depth that others would even desire such that they will want to come get their players. Leverkusen players have got an experience outside the bundesliga already so it wouldn't be totally new playing outside bundesliga next season, they Europe wherein they are doing fyn has got a slight similarities and idea into what the champions league would look like so basically there's no need of going to sought for too many players in order to add depth to the team that would have been necessary if Xabi was not staying back.
Although it will be their first time in a long while but they can still perform so well too if they put their mids to it a d considering the kind of coach they have got, I have almost no fears for Leverkusen rather I feel others would even want to come get players from Leverkusen to add to their own depth because these players are actually building a rear record in European football and it's very commendable I still trust and believe the Leverkusen squad.