Hansi Flick will have difficulty to improve Barcelona defense if he wants to gain success with Barcelona. If he fails to improve defense system for Barcelona, there is very small chance of success there. Furthermore, attacking power of Barcelona was affected a lot last season with decreasing performance of Lewandowski because of ageing. Ageing effect on Robert Lewandowski can become bigger in a new season and this bring another big difficulty to Hansi Flick.
Barcelona's financial status is the issue why they can't get expensive players that make the squad strong like other clubs; they don't have enough money. Although I know that even with current players, they are still trying to do something for themselves, but still, the players are not that good that will give them a trophy. I think there are only a few players in the Barcelona squad that are very good, and only they can do what Barcelona wants.
However, regardless of how good Lewandoski is, I think next season will be difficult for him because next season is going to be a tough one and his age is going to affect his performance. The best solution is to get new players, and since there is not enough money, Fick should be ready for many challenges next season because it is not going to be an easy task for him.