If Setien stays (a big if, depends on beating at least Napoli and not getting annihilated by Bayern, both of which are very unlikely), I see Busquets staying and being a starter. Raki and Vidal sould be on the way out though, whoever the next coach is.
If we manage to get rid of Raki, Vidal and demote to rotational players Busi and Suarez we would be moving in the right direction.
We won't be winning anything for a couple of years but the team can stay competitive, having Messi in the lineup, while rebuilding with new and hangry players.
Pjanic can be a good deal, of all that happens.