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.
If Messi comes back to Barcelona I won't be expecting much from him because when Messi was in Barcelona he played a major role for Barcelona and this time around I don't expect we can still expect that great performance from him because right now he is kind of advanced, he may not give his best to Barcelona.
The possible thing is he may come back to the barcelona but with different status. he may come back to the barcelona as a coach rather than player pretty much the same like what xavi did. Xavi was coming back to the barcelona as a new coach for barcelona rather than become a player again. So many speculators have been saying this thing many times. Im not even thinking messi will back to the barcelona as a player.
His old will become too older to compete in the la liga. I think that would be a good thing to see him as a new coach for bacelona.