I think the key is how you deal with leaks.
And let's face it, some plots just write themselves. They added Jean Grey to some of the ending shots to counter speculation that she was going to die in X-2, but anyone familiar with Jean's story is gonna be able to smell the Phoenix Saga a mile away, even with clues from the first movie.
I don't suppose the internet, of course, had a lot to do with Spock's death in Star Trek 2, but there was enough of a leak to force them to create (as I read in I Am Spock) the simulation scene at the beginning. "Aren't you supposed to be dead?" -- Kirk to Spock
Cameos don't really bother me with the Muppet movie. It's not exactly like the identities are that vital to the plotline. Assuming Grodin really, truly didn't make it in (in the jail scene we see in the trailer, where he would've fit perfectly -- and sorry, I'm a total Piggy/Nicky shipper, LOL), then what seems obvious is that they just pick some other jailed character (who I don't know, sorry to say) to fill the niche.
The plot of this movie can be written with just the general concept of "bringing the Muppets back", particularly if you're attempting to "honor" the Muppets. Even if they don't sing "Just One Person", Kermit's speech in some of the trailers definitely give off that vibe, which means the ending will have not only a large cast of Muppets, but also (finally) Muppet fans to show their solidarity with the Dream.
I still avoid watching that youtube vid someone shot during filming with the ending song or whatever it was. I really don't want to know. I want to experience the joy of that moment fresh in the theaters. However, knowing that Wanda Sykes or whatever is going to be in it doesn't really affect me as much.