While the original Lion King movie was based on "Hamlet," this film clearly takes its inspiration from the play "Rosencrantz and Guildenstern are Dead" which features the events of "Hamlet" through the eyes of two minor characters.
There is a part in which Pumbaa and Timon argue about whether to help Simba or not, and Timon refuses to go with Pumbaa but eventually changes his mind. This part is loosely based in the added part of "Disney's The Lion King" children's novel, written by Gina Ingoglia.
The trivia items below may give away important plot points.
The characters (from not in the movie) that show up in the end are (not in order): Mickey, Donald, Goofy, Stitch, Pocahontas, Peter Pan, Tinkerbell, the Lost Boys, Aladdin, Jasmine, the Genie, Quasimodo, the gargoyles Hugo, Victor, and Lavern, Terk, Baloo, Mowgli, Snow White, all seven Dwarfs, Lady, the Tramp, a hippo from "Fantasia," Belle, the Beast, Mrs. Potts and Chip, Dumbo, Flora, Fauna,and Merriweather from "Sleeping Beauty," Brer Bear, the Mad Hatter, and the March Hare.