Gamasutra has a fairly good editorial on expectations for series and franchises.

Nintendo’s Metroid series changed more considerably when it finally made its way into the foray of 3D on the Gamecube. While past Metroid games had been 2D action platformers using the same viewpoint as its contemporaries, the new Metroid, dubbed Prime by developers Retro Studios, was presented from a first person view, akin to shooters like Quake and Halo.

When the game was first revealed, many long time Metroid fans demanded and explanation for such a drastic shift in the gameplay. Some even declared that Metroid proper was dead, and this new series was nothing more than a shadow of what 3D Metroid could have been.

Retro and Nintendo fired back with a simple explanation: both companies felt that Metroid was not defined by its viewpoint or its graphics. Such changes were only cursory and did not take away from what really made Metroid: a sense of exploration, and isolation. Thematically, the argument was air tight. The series still featured heroine Samus Aran as well as her long time enemies the Space Pirates, and of course the alien metroids. Many still condemned the series, and some outright ignore its current iterations.

For developers, it can be hard to understand what it means for a specific iteration to be part of a series. Many times developers want to take a series in a new direction for the sake of their own sanity, and for the sake of growing the series to be something more. The danger, of course, is in angering fans. Nintendo has been given a tremendous amount of latitude when defining what made a Super Mario game. The series’ progression from Mario Bros., to Super Mario Bros., to even Super Mario Galaxy, the series has time and time again thrown out what it considers the Mario canon and given us a whole new world to explore.

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Perhaps game series with a very consistent make-up from game to game tend to attract fans that expect a specific type of atmosphere and gameplay, and want it over and over again. They consider the series’ synonymous with a certain type of gameplay, story and protagonist. While Super Mario Galaxy seems on its surface like a strong departure from its past entries, each game does still adhere to certain thematic and gameplay related elements far beyond simply ‘jumping on mushrooms with eyes.’

Maybe the key to giving yourself room for reinvention in later iterations is not to adhere particularly to a given concept from the very beginning. By constantly changing what people expect out of a series, it becomes difficult for gamers to complain that some stale and stagnant gameplay mechanic defines the series, and thus allows developers to continue being creative, and reinventing the series their fans love and desire.