The best film series, as with the best book series, didn't start out as series. One good story was told, did well, and then was continued. The creator may have had future plots in mind, and put in plot hooks that could be later played with, but the original tale was complete in its own right. This is the best way to do it... especially in film, where there's a good chance you in fact won't do well enough to get your sequel.

Heck, you could argue that the villain is just a hero who lost.

I had a thought today I don't think I've heard mentioned. Is part of the death of musicals due to the rise of using actual songs as background music? In the episode someone listed Tarzan as one of Disney's musicals. It's not. All the Phil Collins songs are just part of the soundtrack, with no characters actually singing. You get the same emotional effect, speaking the subtext, without having your characters dancing around the screen. From the studio's perspective, it's even better. Old MGM musicals were often created around existing songs to promote record or even sheet music sales. Now you can use any movie to do that!

Thus, we can blame either Butch Cassidy and the Sundance Kid, which had the "Raindrops Keep Falling on my Head" bit, or Lucas for American Graffiti.

Look how close it is to the Earth! Gah! Where are the space arks?!?!

Actually, Toy Story was made because they could do plastic really well, and found a story they could tell.

OK, now that Pixar has fallen, and other animation studios are putting out good films... can we STOP referring to these movies by studio? Name the director. Name the head animator. Whatever. We don't assume all live action films from a studio have a certain quality, and we shouldn't in animation. Especially as the top creative people start to move around, freelance the same way live action directors do. Let's give the actual creators the credit.

Toy Story is one where, really, you could do endless new films if Pixar was willing to recast. Have an entire new bunch of toys in a new house with new problems. Nobody ever likes that, though, especially the money people.

(There was one book series, can't recall if it was Roger Zelazny's Amber books or something else, where each was to have a new main character until the publisher nixed the idea. The Friday the 13th and Halloween movies were to not have continuing monsters either)

There are so few filmed performances that there's little you can generalize about them. Usually they're done at the end of a run, as you said, or are even one off versions. PBS has aired some. Personally, I dislike the ones that are "cinematic", as opposed to just pointing the camera at the stage, mostly because they often are filmed without an audience. I want the actors to feed off the applause.

The lack of original movie musicals may have a bit to do with the problem of doing movies in general. Nobody is going to green light an unknown project, and no creator is going to bring their original show to Hollywood when they have a much better chance of getting it first performed on the stage. Gone are the days when a studio will have in house composers to provide songs for films (and, lets face it, most movie musicals used existing hit songs as a way to sell records, and you can get that now by just playing the song over a scene). I'm sure the reason we don't get musicals in independent films is many of those are from writer/directors, and very few of those are also songwriters, or willing to bring in yet another creator to interfere with their vision.