Overall Hansi Flick's performance will really be seen by everyone next season while coaching Barcelona even though the coach has had a good performance with his old team in the past. However, next season Hansi Flick will handle a more different season with a more competitive team because remembering that the current rivals are also quite strong rivals in La Liga as you said. But what still concerns the Barcelona fans themselves is not the coach, but the players themselves who must be better because Barcelona needs to compete at the front quite consistently next season.
In 2020 Hansi Flick guided Bayern Munich to win the Bundesliga, DFB-Pokal, and Champions League titles. But his job as the coach of the German National team was not fruitful, so he is now back to club coaching. Barcelona has what it takes in terms of quality players to compete in all the major competitions this season. If the German can manage the players at his disposal, they will be able to give Real Madrid a good competition. I don't think he has any Spanish coaching experience which will make his work more complex. Xavi Hernandez was a good coach but he was sacked because of the club's performance in the UEFA Champions League, maybe Hansi might be the manager that will bring back the European lost glory of Barcelona.
That's very different situation with him in Barcelona now. Hansi flick got a legend talent when he was coaching bayern munich at that time, then he is now coming to the squad with so many young talents. We can't really put a lot of expectations to flick's performance in the barceloina.