Dani Alves' replacement in Sergi Roberto provided the heroics for Barcelona this season with not only a last minute goal against PSG at home, but also with his 50-60 meter run in the dying seconds of the El Classico match that set up Messi's winner. He's definitely no Alves, but Sergi has had a decent season this year.
Had Thiago stayed with Barcelona he would have undoubtedly been Iniesta's successor, instead they have had to rely on the likes of Andre Gomes and Denis Suarez to fill in for Iniesta for much of this season. I think the lack of creativity in the final 2/3 of the field has stood out more for Barcelona (with Iniesta out), more so than for what they lost in defense by letting Alves leave, hence both Neymar and Messi (especially) having to play a lot deeper role this season in order to help Barcelona's midfield retain possession. I think this is one of the reasons as to why Neymar's goals have been much lower this season, while at the same time he ended up surpassing Xavi by setting a new record for the amount of assists in the UCL this season.
Also, despite how well Alves' has played this season at Juve, I feel like they have a much bigger problem at the left-back position with Jordi Alba more so than they do at the right-back position with Sergi Roberto. Alba is a lot more defensively flawed (allowing his man to score the header against Deportivo La Coruna which essentially sealed La Liga for Real Madrid, failing to track James Rodriguez in the away El Classico fixture which allowed him to score the 2-2 equalizer, Barca only conceding one goal against PSG with him out the starting line-up when they let in four when he was playing, etc.) and he doesn't link up with Neymar that well at all. I'm hoping he gets replaced or Barca adopt a 3 at the back formation for next season.
And yeah, Dybala is definitely more of a fit for Barcelona than Coutinho who is completely overrated. I think he would thrive in a 3-4-3 formation similar to the one Barcelona used in their home tie against PSG this year. They had a diamond midfield with Busquets in the holding position, had Messi playing as CAM and Rafinha as the right winger. Instead, just insert Dybala in the line-up for Rafinha. If Barca could get another fast young CB to link up with Umtiti (who IMO has been a top 5 defender in Europe this season) and Pique (still great, but is vulnerable on the counter due to his lack of pace) that formation could be real lethal for them.