Sequels, Necessary or the Death of Imagination

With the slew of sequels that have been pouring out of Hollywood and game studios recently I wanted to touch on the idea behind sequels.  For years sequels have been the continuation of something from the original idea.  Kind of like "the continuing adventures of" whatever the original idea was.  Recently released "How to Train Your Dragon 2" was a sequel to the Dreamworks hit and has been one of the few to have broken the barrier between what I think of as "continuation" and "progression".  I think that the key to a great sequel is that the characters and story from the original movie or game needs to progress and show a new aspect of the people in it.  Something that brings you in deeper and wants you to know more about them.  I think that the sequel needs to almost be able to stand alone as a story unto itself.  The characters are introduced in the original and they have a story but the sequel needs to be a new story and deepens the relationship between the core characters.  I think that Dreamworks has figured this formula out because both the Dragon series and the Kung Fu Panda series both follow this formula, almost to the point that the sequel is better than the original.  I think that other movie franchises could stand to take a look at this and learn better how to present and write sequels so that we are not looking at the same stories that have already been told but new adventures that involve the same characters but with a deeper look into them and what makes them so intriguing.
I think that the game community is still figuring out this.  The Assassins Creed series has been falling into this idea of continuation recently.  The first game was awesome, the second game progressed the main character and the story, but then came two sequels that were just continuations.  Their story didn't really progress the main story along, they did introduce some new mechanics and ideas but the story that was at the core kind of got shelved in the back.  They seem to be getting back on track with the recent additions but I wonder when the next continuation will come out.  Other series have gotten stuck in this loop and it usually ends up that they are holed into a very small niche that only the most hardcore play.  A good example of this is the Left 4 Dead games from Valve.  The first one was great, it was different and fresh and was a ton of fun to play.  But then they came out with the second and, aside from a few new quirks, was the same game.  The interest to play fizzled out and now they are played by the most hardcore fans and are largely forgotten by the larger community.  I think that the new sequels that are coming out for various games will also suffer the same fate if developers and writers do not start to look at the progression of their stories instead of the continuation of them.
As I watch more sequels come out to games it makes me wonder if people are losing imagination that they just build on what has come before or if they like the originals so much they want to add their creative touch to it and feel like they are part of something that they love.  I have no issue with wanting to be a part of something that is connected to your passion but don't try and duplicate it with new bells and whistles.  Take what you love and make your own unique mark on it and progress, don't continue.