One of the remarkable things about The Simpsons was how quickly the show built enough cultural cache to attract big-name guest stars. Back in the first season, Albert Brooks played French bowling instructor Jacques on Life in the Fast Lane. Since then, celebrities have continued to flock to the series, to the point that best simpsons guest stars include some of the most famous and influential cultural figures of the last three decades.

From A-list actors like Michelle Pfeiffer and Dustin Hoffman to music legends like Johnny Cash, the show has drawn talent from across the cultural spectrum — though Bruce Springsteen still refuses to appear on The Simpsons. But even Bruce Springsteen isn't as heavy as two of producer Al Jean's dreams. These two characters are just as prestigious as anyone who has appeared on The Simpsons, but there are a few problems preventing them from appearing - the biggest being that they've both died.

There are many why celebrities agreed to guest on "The Simpsons" even at a time when most people agree that the series is long past its prime. Aside from the fact that lending their voice is often much easier than appearing for a live-action cameo, the show still retains some of its cultural appeal. Having a cameo or guest appearance on The Simpsons is something of a right of passage for any big name at this point, regardless of the show's quality. Unfortunately, longtime producer and host Al Jean would never be able to get his top guest stars.

Jean joined The Simpsons for its first season back in 1989, becoming showrunner alongside his writing partner Mike Reiss for the third and fourth seasons. Although Jean left the show after its fourth season, he returned in season 10 and remained with the series until its 33rd season. But even in the first four years, he saw Elizabeth Taylor, Michael Jackson and even Ringo Starr grace the show.

But it turns out that another member of the Beatles will forever be Jean's ultimate guest star that she never was. Talking to NME, The former show host said: "A lot of those who I would have said would have been a dream never came, like John Lennon." The legendary songwriter died in 1980, nearly a decade before The Simpsons aired, so that particular guest appearance was ruled out even before the show began. But Jean's second choice could very well have made it to the show in his lifetime, making him a truly runaway Simpsons guest star.

Although The Simpsons did get Buzz Aldrin, the second man to ever walk on the moon, they never got to get the first. It's a shame considering Neil Armstrong was alive until 2012 - at the start of the series - and is also one of Al Jean's dreams.

Fans of the show may remember Armstrong appearing in the Season 10 episode "Mayored to the Mob," where he shows up at a booth at the Springfield Bi-Mon-Sci-Fi-Con only to be completely ignored by the attendees. That particular look was actually voiced by Homer voice actor Dan Castellaneta. Also, John Lennon appeared on the show, but his voice was provided by cast member Hank Azaria.

Elsewhere, Armstrong's actual voice appeared on the show, but only via archival audio of his famous moon landing quote, "One small step for man, one giant leap for mankind." In that sense, Jean at least in some way lowered his dream. The showrunner also revealed that the other "Holy Grail" star would be president of the United States, but all the presidents they approached said no, doing this. the only type of guest star The Simpsons could never land - Well, them and Bruce Springsteen.