Square Enix have a big MMO. It's called Final Fantasy XI. But FFXI is getting a little long in the tooth, so it's no surprise to hear Square Enix are still working on a successor.
Talk of the company's post-FFXI MMO has been around for years now, but during an address to investors last month, Square Enix boss Yoichi Wada re-affirmed that while Final Fantasy XI was still quite profitable for the company, it couldn't last forever.
Speaking of this "transition", Wada says that, as FFXI winds down, "the question is where do we go from here, and we are currently developing a post-FFXI flagship title". Interesting that he calls it a "flagship" title, suggesting that instead of just switching to a more casual-friendly MMO like the company's existing Nicotto Town, it'll be something more substantial.
Final Fantasy XIV, perhaps? It's not like that'd be without precedent...