Most of the characters on "The Simpsons" don't stray too far from their home base. Through the seasons, character additions have been more common than character role shifts -- an exception being Barney Gumble's transition to sobriety after his transformation in Season 11's "Days of Wine and D'oh'ses."
When you think about it, most characters in the realm of the cartoon or sitcom don't normally shift roles much. The audience becomes comfortable with the characters and get used to their place within the show's framework. I've always wondered what made "The Simpsons" so different. The majority of their characters are one-dimensional and stereotyped. Yet they've endured for more than 20 years. What then struck me was the shear number of characters the series has added since its inception. When compared to other cartoon shows and sitcoms, "The Simpsons" has an absurd number of characters, even if most are one dimensional.
The enormous cast of characters has helped keep the show fresh for so many years, despite its downturn in recent seasons. The vast number of characters has a twofold benefit to the show: we don't tire of the characters because they aren't consistently focused on enough to grow boring; and it gives the writers plenty of directions for plot mining.
This ever-growing cast of Springfieldians does have a downside. Because there is such a wide array of side characters, when a plotline does deal with them -- see Superintendent Chalmers in Season 23's "Bart Stops to Smell the Roosevelts" -- it seems random, stretched and forced. It's not that the plot itself isn't any good (Chalmers taking on a challenge from Principal Skinner to actually "teach" Bart Simpson), it's that in the previous 22 seasons, we have never been given the inclination that Superintendent Chalmers actually cares about the children in the public school system. To suddenly see him in this conscience tugging, teaching role is jarring and eyebrow-raising.
With so many characters around Springfield, many lay dormant for episodes, if not seasons, at a time. Their appearances are welcomed by fans, but when they are thrust into a lead role, their one-dimensional side role doesn't have enough meat to revolve an entire episode around them. So, the writers are forced to expand the character, thus challenging or straight up changing the character's construct -- a construct they've spent 20-plus seasons developing.
It's a strange trade off. The very thing that I believe has made "The Simpsons" so enduring over the years is the very same thing that is leading to its decline. No matter how long it takes for the show to end and how many characters will have been randomly expanded, it will have been one heck of a ride in our Elec-Taurus when it's all said and done.