At the recent Final Fantasy XI Fan Festival, Gamasutra's Emily Balistrieri got a chance to sit down for tea and biscuits with Square Enix VP Hiromichi Tanaka. They chatted about all kinds of stuff that will bore anyone who's not a rabid FFXI player to death, but this bit, this bit's a little more interesting:

As you might already know, members from the current FFXI team are working on a next generation MMO, but it still hasn't been decided whether that's going to be a game in the FF series, a continuation, or a totally new game. We are working on it, but it hasn't been decided that far yet.

So it'll just slot right in, regardless of whether it's Final Fantasy or not? Good to see Square maintaining their long, storied tradition of variety in character and world design, then.
Event Recap: Final Fantasy XI Fan Festival 2007 [Gamasutra]