Bayern Leverkusen will not fall off, they're also making good signatures gradually to add depth. They're making too much, but having Xabi Alonso is enough. I don't know though, but expecting too much from next season is not what I'm saying but they won't be off the first four clubs in the standing.
So far I haven't read any Leverkusen players who are definitely leaving. But some very strong names and are being targeted by many clubs may be Frimpong, Wirtz, Kossounou, Palacios and Boniface. On the Transfermarkt website their prices go up quite a lot because of the good looks. So Xabi must certainly be able to hold back some key players for next season. Even the best thing is to add players by releasing some players who don't have a big contribution to the team. But if that is not done then it may be difficult for Leverkusan to defend the Bundesliga title next season.