The most important thing is to hire the right manager. Oscar Garcia is by far my first choice, followed by Maurizio Sarri but none of the other rumored managers excite me at all.
We need to address the situation at right back. I'm probably in the minority on this but I don't think we necessarily need to sign a new one. Roberto didn't really work out and is clearly much more valuable in the midfield. But before his injury, Vidal was really starting to impress and for a couple weeks we looked dangerous on the right wing. He's certainly no Dani Alves but I still think he's good enough to be our first choice right back for a year or two until we have a more long term replacement. And I believe that long term replacement is Sergi Palencia. I think it's time to stop selling our youth products and start giving them opportunities in the first team. Palencia has led Barca B to a first place finish in their group and possible promotion. He deserves to be promoted and is already good enough to step in when Vidal rests. But if the board doesn't have enough faith in Palencia and Vidal and wants to buy someone, Bellerin would be my choice.
With Mathieu most likely leaving and possibly Mascherano too, we will need to strengthen our center back depth behind Pique and Umtiti. Marlon is the obvious option to replace Mathieu. There's not much else to say about it as most fans are in agreement here. Mascherano, I'm much more torn on. On one hand it does make sense to sell a 32 year old player who is coming off the bench to play out of his preferred position. But on the other hand, he can still contribute a lot and maybe most importantly he is the best leader on the team. As much as I would hate to see him go, it has to happen eventually and now might be the time. If he is sold, my choice for a replacement would be Mauricio Lemos. He is young and has huge potential but he could be bought for a decent price.
Our midfield certainly needs some changes but I don't think buying another center mid is necessary. In my opinion the two things hurting our midfield most this year are our right back and our manager, not our midfielders. With those two things fixed, our midfield will have better shape and possession. As for personnel, Iniesta is still world class but his minutes will continue to diminish. Denis needs to be taking most of those minutes because he is plays very similar to Iniesta and can be his long term replacement. Rakitic is still looking good and has probably been our best center mid this season. He still deserves to be in our best XI but Sergi Roberto provides a more than capable replacement. The only place where we need more depth is behind Busquets. Nobody in the current squad can effectively fill in when Busquets is resting or injured so I would bring Samper back from loan to provide depth. That's all I really think we need in the midfield. Aleña or Rafinha can always step in if there are injuries but I think Gomes has to go. He has shown some good things but I just don't see a future for him at Barca. I didn't think he was a good fit when we signed him and I haven't really seen anything to change my mind. With all the young talented midfielders at Barca someone has to go and Gomes is the obvious choice.
Although it's a rather minor issue, we should address wing depth. Right now our opinions are Paco, Arda, Rafinha, and Denis. Paco is a striker and should be getting most of his minutes there, Arda will probably be sold this summer, and Rafinha and Denis are naturally midfielders. Bringing Munir back from loan is the best option in my opinion. I wouldn't mind if we bought back Deulofeu too, but I don't think that's really necessary since we do have Rafinha.