With Square Enix's willingness to take inspiration from Western influences, it's definitely plausible they could use a similar approach as the Fallout franchise to great success.
The level of interactivity in Fallout and its massive open world could fit well in the Final Fantasy universe. I especially would like to see how the companion system in Fallout would work in the game, and what the community would do with modding opportunities.
As long as the developers are careful not to pander too much to a Western audience, and keep the fundamentals of what makes a Final Fantasy game, there is no reason not to try and appeal to both audiences to continue the series global success.
Whatever they decide to do, I'm confident it will be another experience worth playing.