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.