If Messi comes back to Barcelona next season I'm pretty sure that he would do his best for the team. I don't think that Xavi would change his strategies to build a game based on him. But he would surely find the right place for him in the starting eleven.
His current performance level at PSG is definitely enough for the team. As long as he keeps this up regularly they will stay strong against every team they face in the Champions League. After having a very productive season I wonder if Barcelona will try their luck by making an offer. Actually they have the biggest credit for Messi already as long as they agree terms with him about a contract.
with great consideration, Barcelona should be able to bring Messi home. but that would go against Barca's current finances. although quite good with the signing of players in the summer transfer market. but Barca also had to release some of their players to cover their finances.
brought Messi from a rich club and was concerned with business. I don't think it will be easy for Barcelona to get Messi again. except with Barcelona's ability to reach an agreement with PSG.