It's so painfully obvious that it's going to be Glenn that it's, well, painful.
The whole point of the dumpster scene was to "trick" the audience into thinking he had plot immunity and that they'd never kill him. If they had done it pretty much any other time in the past, and not part of the same storyline in which everyone already knows he was going to be killed because of the comics, it might have actually worked. But it didn't here. Again: It was painfully obvious what they were doing even when they did it.
Doubly so since they basically did the same thing with Abraham by substituting Denise in just a few episodes prior to the finale. "Look audiences, we're totally going to change things! And, and, and you know Glenn is safe because of the whole dumpster thing! (Mouhaha, we're so fooling them, Kirky!)"
Much as people seem to think the writing on the show is brilliant (I enjoy it tremendously, but I wouldn't ever call it that), the writers are about as subtle as a brick to the face when it comes to a lot of things.