I don't think Messi is interested in staying Barcelona anymore.
Who knows! How if Barcelona management offers him to be an assistant coach if he stayed. Then, he has a chance to be a Barcelona coach in the future. I think it is very possible and hard to refuse.
Well, first of all we don't, does Messi wants to become coach after he will retire. He never said anything about it. And after all, I think it's possible that he will stay in Barcelona after end of this season. Next summer there will be Barcelona president elections, so Messi decision will depend on election results I think.