Barcelona should not think about their failure too much right now because they started the season really badly and they ended up finishing in the top two, which is an achievement in itself in my opinion.
Barcelona is going to concentrate on doing well in La Liga next season. They will also be trying to do well in the Champions League but I don't know if they will be able to achieve any great thing in the Champions League with the current squad. I think they should concentrate on La Liga right now and over time they will make their squad stronger through some transfers and they will be able to concentrate on the Champions League then.
They do not have to be great right now this season, they had a horrible start of the season that is true, but not like they fixed all of their problems. It is true that Xavi has been much better than Koeman but I am sure that koeman is furius and yelling at TV screen "if you gave me those players like auby+traore+torres then I would had better results too!!!" and that is why the real difference will always be roster change.
Barcelona needs to get better players and all that, but they do not have financial means to do so, which means that maybe it will take longer than just one year for them to turn things around and start being champions everywhere.