Bayer Leverkusen is one of the clubs that opponents fear in the Bundesliga. Not all clubs can beat Laverkusen easily because they were at the top of the standings last year with good performance. It would be really good, as you said, that Laverkusen should add several senior players and star players, especially strikers and defensive midfielders, so that they can be more confident of winning this season.
The Leverkusen team is still strong enough to defeat any opponent this season, in my opinion, Xabi Alonso has signed about four players during the transfer window, so I don't see anything wrong with them, but we shouldn't expect them to play as well as they did last year, as they won the Bundesliga title without losing any game, they have only played two games this season, winning one and lose one, It would be wrong to assume that they won't win the league, I think Xabi Alonso has everything under control and they will return to their winning ways.
Furthermore, Leverkusen will face tough rivals this season, since they will compete in the Champions League, where they need to win every game. I won't be surprised or embarrassed if Leverkusen's performance did not go further this season, they had already achieved a lot this season, and it is not easy to win a treble, but I had strong confidence that Leverkusen was not done yet, and we would see more from them this season.