Final Fantasy was once a game in thrall to tradition; in the past 20 years, it has become an on-going revolution. Final Fantasy IX was the last mainline instalment to make use of the Active Time Battle system, which had carried Square's flagship series for no less than six games across two console generations.
Since then, no numbered Final Fantasy has given us the same battle system twice. Each entry has been an experiment, obliged by changing audience expectations and company dynamics such as the promotion or loss of key staff. All have sought to stand apart, but much as Active Time Battle isn't a clean departure from the turn-based systems that preceded it, so most Final Fantasies circle back to ATB in some way, striving to rebottle the lightning.