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.
Maybe you will turn out to be right about it.
As I said before, Xavi has built a completely new system for the team. There is no such thing as building the game over one player anymore. Because they don't even have any player like Messi now so there is no meaning in keeping that style. Xavi wouldn't change his system just because Messi came. Maybe he would adapt to the system if he returns as a player. But maybe things will be as you said and we will see him working with the team as a coach. Seeing Xavi and Messi together is enough for me to get more excited about this team.