The problem with analyzing the future of Lionel Messi is that, after 20 years, his relationship with Barcelona lies somewhere between the professional and the romantic. You can use reason and logic on the first part to come up with plausible scenarios. You can't account for the second, partly because it's intensely personal (and this is perhaps the most private superstar in recent sports history) and partly because it's irrational and mutable, in the way emotional relationships so often are.
So let's do the comparatively easier rational part first, shall we?
Messi signed a contract that expires on June 30, 2021.