Its doubtful, IX is not exactly a success compared to its other PS1 brothers, not to mention its a game that was developed by some old time core FF team members who most of them left Square shortly after. As oppose to IV and VII which both still have some core members from the original games development still hanging around and still in love with the games.

Even if most of the original team was interested in doing another title, that hasn't stopped SE from not taking action, one look at the Chrono franchise is proof of that as three of the most important members of that franchise would jump at the chance to make a new game but SE still hasn't green lighted the project.