I'd be decidedly surprised if Square Enix didn't make a new MMO somehow based in the Final Fantasy universe considering the popularity of the franchise. It's very possible that when FFXI producer Hiromichi Tanaka said, "-we're not going to make something that'll be exactly like FFXI" He merely meant that mechanically speaking the new game would be very different. Which quite honestly should be a given considering the somewhat esoteric and now archaic control/menu system featured in their first game. Also, it would be nice to play a Final Fantasy MMO that wasn't so harsh on players who died -- losing experience is a deal-breaker for many players these days.
I'm a big fan of Vana'diel and its design, but at the same time I would fully welcome an MMO set in the world of Ivalice or even someplace completely new. Although, the more I've thought about it the more I've realized that Ivalice is setup perfectly for an MMO. You've got multiple races, each with very strong defining aptitudes, a long history, lots of different nations and a very different look. Plus, Final Fantasy XII practically was an MMO in many regards. I'm not about to take any wild guesses as to how combat and such will work in whatever game the FFXI team is working on now, but I wouldn't be surprised to find out that the team paid attention to what people thought of the control system in FFXII.
As far as the settings goes, Vana'diel and Ivalice strike me as two very different worlds. Where Vana'diel evokes a strong classic Final Fantasy feel, Ivalice harkens to the renaissance era philosophy in overall aesthetics and design. Not to mention that the fanbase for Ivalice games has been both vocal and ever-growing since Final Fantasy Tactics released on the PS1. My point is that making Ivalice the setting would fall into the logical concept of keeping the original FFXI and this new MMO similar-yet-different from each other. I still wouldn't be opposed to creating another original world like Vana'diel, either.
This made me wonder if we're due for a curveball from SquareEnix: A futuristic Final Fantasy MMO. While I'm sure this would immediately turn off a lot of fans, I do also think there is a large group that would be all for a game set in a world not unlike FFVI or even FFVII. My own personal feelings on a steampunk MMO are already known. Just thinking about a big AAA steampunk Final Fantasy MMO makes my palms just a little sweaty. I'm pretty sure we'll see an announcement sometime this year, so at least the wait won't be very long. All I know is that if it's a Final Fantasy MMO, I'd better get to roll a Moogle character this time around!