I would way rather them make a sequel to a game they intended to add on to then have sequels to self contained games.

Though I don't appreciate leaving a game inadequately developed to leave room for the sequel (for the record I attribute FF13's lack of extrapolation to ineptitude, not holding out), if it is written properly I don't see why we can't have planned sequels.

In the last decade FF titles (an admittedly small sample) seem to have gone away from the 'big world' concept they used to have. In the old titles before you go and fight the last boss you have basically mastered every area of the world, and are the most bad ass thing on the planet sans the last boss. Having the cast of FF4-9 face off against a new foe is a stupid concept, as you have been everywhere and know there isn't a hidden evil empire over the horizon, and even if there was you are strong enough to handle it. (I absolutely DESPISE having a sequel where your lv99 own everything party starts again at lv1.)

In a game like FF12 or 13 there is still a ton of unexplored world. It actualy makes sense that there could be threats out there greater than the party has experienced before. Especially in the case of FF13 where (SPOILER)they aren't l'Cie any more. So long as they give past party members the dignity and relative strength they deserve it could work quite well.