8. Fantasia 2000 (1999) - 59 Years
I'm going to star this list with what I could confirm as the longest gap between sequels, which is marked at 59 years. The original Fantasia back in 1940 was risky business for Walt Disney and company. It wanted to blend a sense of artsy classical music with mainstream animation to prove that art is not only for the high-class and rich, it was for everyone. Technically, the film is amazing. It is beautiful and momentous. Critically, it went one way or another. Financially, Fantasia was not a money maker. It was indeed a risk. So what could Disney's nephew Roy, the one who pushed to get the sequel made, hoped to accomplish with a sequel six decades later? It's hard to say, maybe a sense of appreciation of a dissolving medium such as hand-drawn animation.
Was it worth it? Given that when people think of Disney animated films they usually include Snow White, Peter Pan, or The Little Mermaid but never Fantasia 2000, that could really give the answer. It's not that Fantasia 2000 is not part of the animated canon. It is that most people forget or don't even know that it exists. For the legacy that the original has, you don't see much from the sequel outside of the cover art that's included in the DVD/Blu-Ray combo releases of the two films. It may have seemed like worth making at the time but it is difficult to believe many wanted Fantasia 2000 to happen so badly.