In any update to the ride there will be huge debate as to the movies to choose, but also which are plausible to pull off, or Disney has the rights to do. In all truth, Gump would be easy enough and help represent the career of Tom Hanks. Gump was a big hit, and did inspire more then Bubba Gump Shrimp. Weird Al has a song about the movie. It also is the iconic performance for Hanks. Don't get me wrong I'm not a huge Gump fan, but it is considered a classic, and was in AFI's Top 100 movies of all-time, something Tarzan, Alien, and Public Enemy can't say. Also, though I hate them, Gump did win the Academy Award for best actor, and best film. Him on a bench with the feather blowing around might actually be cool if done right, and it could be a small section. No disrespect though to those other films.
Lots of movies can have a case made for them. I think one thing should be iconic images, like Kong on the Empire State Building, or ET flying across the moon (those obviously are owned by Universal, but they can and should be in the montage.) I like Some Like it Hot, and AFI named it the greatest comedy in the history of American Movies, but it would be difficult to come up with the right scene to represent that classic movie. That's why I've mentioned Marilyn's skirt blowing scene, as it is one of the most iconic moments in the history of cinema. Heck, if you wanted a Disney moment, Lady and the Tramp eating spaghetti to represent romance. Looking at Hank's career, I can only really come up with Gump, or the piano in Big as things to represent him, though Toy Story might have possibilities.
What's a Great movie though? Dark Knight? Wall-E? Die Hard? Avengers? Avatar? Jurassic Park? Sunset Boulevard? Tarzan? Robin Hood? Silence of the Lambs? All About Eve? Sound of Music? Shawshank? Animal House? I will unquestionably admit, it takes time to earn the stature of Great Movie. The first year the Library of Congress collected films for it's library they chose 25 films, and the Godfather didn't make it? Clearly personal bias comes in, and some may hate some of the movies already represented. The benefit some of those have is that they did change movie history, though Some Like it Hot, Psycho, and Jaws changed movies as well.
I always like Tarzan the character, the concept etc. It is important to have a adventure type show represented, but much of what people are saying about the impact of the movie Tarzan, actually comes from the books, pulps, comics etc by Edgar Rice Burroughs (though the yell is from the movie, which is famous). It'd be like saying the Harry Potter movies were completely responsible for the cultural status of Potter, forgetting the books which were every bit as big. Tarzan the character has been done so many times and so many versions, I have no real problem with saying it's a great movie, but I could argue Gump was every bit as great from an artistic and historic standpoint.