Simpsons showrunner Al Jean recently spoke at Fox's Sunday panels at the Television Critics Association summer press tour and teased an upcoming episode that will feature the death of a popular character. The episode will kick off the 26th season on September 28th and is titled "A Clown in the Dumps",
"It turned out bigger than we thought it was going to be. It’s not going to be this bloodbath where they all get murdered."
Jean also gave a hint saying that the voice actor who does the character has won an Emmy for their work on the show. That pretty much narrows it down to two options, Krusty the Klown who's voiced by veteran cast member Dan Castanella or Krusty's father Rabbi Krustofsky who is voiced by legendary comedian Jackie Mason. To say Rabbi Krustofsky is major character is a bit of a stretch. Then again, you never know. Krusty faked his death once already but Al Jean makes it seem like once this character dies it's forever. Hollywood Reporter has the story