Series Loyalty and Straying from the Path

Gamasutra has kicked up an interesting piece called "Series Loyalty and Straying from the Path", which attempts to define how far developers can experiment with a video game franchise without moving too far away from its roots.
What really is the common strand that brings a series together? What defines a game as part of the main continuity? Final Fantasy defined its main entries by high fantasy, as well as thematic story elements such as the crystals, and a struggle between light and dark. Final Fantasy VII changed a lot of this, by introducing steam punk to the mix, and largely doing away with the crystal premise entirely (which had been missing in previous iterations as well). The conflict became far more personal, and much of the high fantasy disappeared from the series.

The high fantasy in the Final Fantasy series reared its head again in Final Fantasy IX for the Playstation 1, which was designed as a Final Fantasy for traditionalists, and even reintroduced the crystals to the storyline. Final Fantasy XI was an even larger departure from the other entries, as it retained the thematic characteristics of the story, but the game itself was no longer a single player RPG, but a Massively Multiplayer Online RPG, in the vein of World of Warcraft.