It is fair to say that when someone looks at a gaming series, like Grand Theft Auto for example, they go in with certain expectations as to what the game is going to be. Someone who played Grand Theft Auto IV is going to have at least some idea of what to expect with Grand Theft Auto V.
Many franchises, however, suffer a problem that comes from when either a publisher or developer suddenly thinks a game becomes too stale (although they have little to base that on), which then causes the franchise to take an unexpected change. In some cases it becomes a matter of debate about whether that game that has changed the series should even be included in the series in the first place, due to the fundamental changes.
For example, many franchises lately have turned toward the idea of making their own MMO, but really have no reason to do so. Take the Elder Scrolls Online, for example. Skyrim was massively successful, there is absolutely no doubt about it. Why then was there a feeling for such a significant shift in gameplay to an MMO in the series?
If we are to look at developers as strictly businesses out to make money, where did the compulsion come from to change The Elder Scrolls series that significantly? Skyrim made a lot of money, selling over 20 million copies as of June of 2013.
Success monetarily doesn't mean that Skyrim was without its flaws and without a need for some kind of change to the way the game was played to help freshen it up, but the series did not need something that essentially changed the genre of game it is in. Some kind of update to pre-existing systems of The Elder Scrolls series would be a good change, but not something so significant to change the game altogether.
Now, it is not fair to completely condemn The Elder Scrolls for this change as it was clear from the beginning of The Elder Scrolls Online that it was not something that players should see as a continuance of the series in the same fashion as previous titles. However, The Elder Scrolls did serve as a good example of the kind of unnecessary change I am talking about.
That is not the only kind of change either. Look at the Assassin's Creed series. There was a change in the newest iteration, and one that many would argue was good, but is it fair to say that the game changed, for the better, what was at the core of the series? No.
Assassin's Creed IV brought with it the addition of the ship and all of the gameplay elements that came with it, like the naval combat. What's at the core of the game though is the act of being an assassin. The assassinations, running through cities, trying to be stealthy, and the occasional fracas with guards and other enemies.
Most of that went largely unchanged from what it has been throughout the entire series. The introduction of a ship/naval system would have made sense if it were complementary to what existed in the Assassin's Creed series previously - but it doesn't. At best, you could argue that the player involves his/herself into the naval combat when boarding the other ship, using the combat system that has been with the series since its inception.
However, that is an unfair assessment as that is just one aspect of the Assassin's Creed series, the swordplay - which, with a series that promises an assassin-like protagonist, you would think a more subtle approach to combat would be given far greater emphasis.
The only thing the inclusion of the ship and naval combat complements is the setting of Assassin's Creed IV. That new system brought a necessary distraction from the largely unchanged, and stale, core components of the Assassin's Creed series.
While Assassin's Creed did bring with it change, it was the kind of change that made the series a nearly different beast than what it was before.
Another quick example would be Splinter Cell: Conviction. At its most base elements, you can still argue it a Splinter Cell game, but Conviction was far more focused on the action part of the game than what the series built itself on, which was its stealth gameplay. Luckily, Splinter Cell: Blacklist has taken it back to its roots.
This doesn't mean that any of these games discussed here are bad, but that some condemnation should come as the series are manipulating player expectation.
Many examples of good change exist, especially in "series' of only 2-3 games, but that would be a bit unfair to look at those, as they have not lasted long enough for a developer to believe them "stale." Many great sequels to games exist out there like Portal 2, Batman: Arkham City, and Mass Effect 2.
A good example of a series that constantly changes to freshen itself up, while at the same time staying true to what we see it as, is Mario Kart. Each new iteration has changed something about the game to add a new element to the gameplay, while still keeping the core elements largely the same. Wikipedia has a good list of the changes in the series.
Another great example would be the Final Fantasy series, up until Final Fantasy X (so, not counting XI-XV now). Final Fantasy takes a different approach than other series, where the genre stays largely the same for each game (except for the few in the series recently), but the systems within each game are vastly different. For example, in each game the combat and gear systems have something unique to them not found in any other game in the series. So, Final Fantasy is a good example of a series that isn't building one on top of the other, vertically one could say, but takes a more horizontal approach by creating largely different games with a common thread throughout them.
That is something we must always be aware of as well: how different series approach what makes their series unique. As something like Mario Kart builds on top of itself, and something like Final Fantasy expands outward.
To reemphasize, the games discussed here are not inherently good or bad because of the change that may or may not complement a series.
Regardless of quality, this is important to discuss and examine as the concept behind a game franchise, and the creation of one, is predicated on players buying into whatever unique element that franchise has to attract players to it. If a series suddenly switches the emphasis away from what created the franchise, then it is unfair to call it a part of that franchise in the first place.
At that point, player expectation has been manipulated into thinking the game they are about to play will have that same draw that drew them to the franchise, when in reality it may be something wholly different - good or bad.