They won't do anything.

Square Enix all but ignored the 20th anniversary of Final Fantasy VIII, but you could argue that it was a different circumstance because they lost the original source code and couldn't port it to current-gen consoles as a way to celebrate the milestone.

In the case of Final Fantasy IX, the game has already been ported to the PS4, Xbox One, Switch, PC, iOS and Android. There isn't anything new that they could do.