Given what Eusebius says about Mark ending at 16:8 ("That is where the text does end, in almost all copies of the gospel according to Mark," with the rest having longer endings), I doubt there is any manuscript evidence for it, but if Mark had originally ended at 16:7, it would be in keeping with what Paul says in 1 Cor. 15:4-5 ("that he was raised on the third day according to the Scriptures,
and that he appeared to Cephas and then to the Twelve"). It would also be more hopeful and not so disparaging to Jesus' Jewish followers.
It seems like every verse after 16:7 goes against the grain of Paul and Jesus and disparages Jesus' followers and moves on to the Gentile Mission, which seems kind of fishy to me. And while that's all I have to go on, it's the only option that doesn't contradict Paul or what Jesus says in 14:27-8, and I gather the consensus is that the author of Mark knew Paul's letters, so 16:7 seems like a suitable ending to me.
16:5 When they entered the tomb, they saw a young man dressed in a white robe sitting on the right side, and they were alarmed. 6But he said to them, “Do not be alarmed. You are looking for Jesus the Nazarene, who was crucified. He has risen! He is not here! See the place where they put him. But go, tell his disciples and Peter, ‘He is going ahead of you into Galilee. There you will see him, just as he told you.’ ”
This just
seems like a great place for Mark to end since it's in keeping with Paul (that Jesus appeared to Peter and the Twelve) and with what Jesus says in 14:27-28 (that the Twelve will see him in Galilee: “
You will all [which includes Judas] fall away ... But after I have risen, I will go ahead of you [which logically
also includes Judas] into Galilee”). If Jesus doesn't appear to Peter and the Twelve (including Judas), then Jesus would be a liar, and that is what 16:8-20 makes him. Only ending Mark at 16:7 resolves this and is in line with what Paul and Jesus say.
So my guess is that the idea that Jesus appeared to Judas (as suggested by 14:27-28) had to go (thus the reference to "the Eleven" in 16:14), and the Gentile Mission had to take precedence over Jesus' Jewish followers (who are rebuked for "their unbelief and hardness of heart"). Maybe I'm wrong (and I gather the manuscripts don't support it), but for these reasons 16:8-20 seems fishy and easy to dispense with.