Ten Hag has already said his first words "We want to play attractive, dominant and exciting football". But we don't know yet how he will act in the transfer window, the departure of star players will definitely affect Leverkusen performance next season. Leverkusen will definitely be quite active in recruiting new players, Ten Hag will try to build his ideal squad depth, I think he will spend a lot of budget like at Man United.
Leverkusen only managed to reach the last 16 of the Champions League this season, Ten Hag might be able to take it further next season. But all that will not be easy, Ten Hag must go through an adaptation phase, which of course will take time. In the meantime, I think Ten Hag can easily understand the dynamics of competition in the Bundesliga, they will compete with Bayern Munchen, and I hope Dortmund will also be involved in the competition for trophies next season.
Certainly the departure of some crucial players in the team would have a negative effect on the team but if Ten Hag and his team are able to sign the right players they could even further strengthen and improve their squad to be good enough to battle for the league title next season. I hope he doesn't repeat same transfer strategy he used at United here with Leverkusen.