The reason AC and MW get games within a year has a lot to do with the fact that they have interconnected storylines. Each game sequentially adds more story to the series, and in all likelyhood, work on the next game has already started by the time the newest one is released. (i.e. Ubisoft realized they couldn't finsh Ezio's story by the end of AC: Brotherhood, and started working on AC: Revelations while Brotherhood was still in development.)
Then you have franchises like Halo. It'll be another year before we get to see how the new trilogy turns out, (2 yrs after Halo: Reach.) We have the Anniversary edition, which has an HD facelift, and some new features, but it's still the same original game from ten years ago.
Japanese game studios have shown in the past that they can put out great titles in a similar timeframe (the .hack and Xenosaga series being two examples)
I'm not sure a game a year would work well with Final Fantasy though, mostly since every numbered title is essentially a stand-alone storyline. That means you are essentially trying to come up with an entirely new storyline/lore for every title, that's not easy to do. In addition, most titles in the series have different game mechanics. XI and XII had similar battle engines for example, but XII also introduced Gambits and a License board, and XIII was radically different from either of those two titles.
In all likelyhood, there may be further expansion of previous titles, since it is somewhat easier to expand on exsisting canon.
(In a literary example, Ender's game was originally a short story, then it became a full novel that expanded into a tetralogy, not including the Shadow series and Ender in Exile.)
