It’s obvious to assume that Mark, a skilled and thoughtful writer, did not mean to end the book thus, and that an original ending has been lost. But that is where the story gets puzzling. If we assume the standard theory of the composition of the gospels, then Mark wrote about 70. Perhaps a quarter century afterwards, his book was used by both Matthew and Luke, who incorporated virtually his whole text, and it is clear that neither author knew any other or fuller ending. If an ending was lost, it vanished very early indeed, if it was ever written. At least by the second century, various editors added their own conclusions to satisfy what they felt to be the gaping hole at the end of Mark, and one survives in the KJV as Mark 16.8-20. ...
If it really was meant to end at 16:8, Mark may be the greatest anti-Christian, anti-Jesus movement, tract ever written. It could scarcely have been so highly regarded as it was, still less accepted as the basis of other traditions. ...
It’s very likely indeed that the next scene would have been something very much like John 21, with a Resurrection appearance (a) to Peter (b) in Galilee. If the ending of Mark’s gospel actually did exist and then was lost, this is presumably just what it would have looked like. (Rudolf Bultmann was one famous scholar who argued this).
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First of all, Mark is not a "skilled" writer. His Greek is everywhere clumsy, as John C. Meagher noted long ago, may he rest in peace. The almost laughable uses of kai, euthus, and gar represent just three examples well known to students of Mark's style, if it can be dignified as style. Mark has some facility with Greek as a second language, and evidently is using it to communicate to a special audience of similarly situated individuals, perhaps in Rome.
Matthew and Luke "incorporated virtually his whole text", but not in the verbatim manner this suggests. Those authors frequently change details of Mark's content, and improve upon his Greek significantly. This means they are correcting Mark as much as they are using him.
The absence of a proper ending which then fails to show up in Matthew and Luke indeed shows the early absence of the ending, but it is also another reason why Matthew and Luke felt the need to write their own compositions. They believed Mark to be inadequate, and inadequate on a number of levels. Luke especially shares this attitude about the inadequacy of the previous accounts with which he is familiar, and presumably "John" would not have written his account subsequent to the Synoptics if he thought they were adequate, else he had not departed from them so radically.
The use of Mark as architecture for composition by Matthew and Luke pays respect to Mark, it is granted, but was Mark really "so highly regarded"? The dearth of early manuscript evidence for Mark speaks against it. A highly regarded gospel would be copied more often from the beginning, and more copies would survive from that time than do.
Finally, the suggestion that what we have at the end of John, as hinted at by the Gospel of Peter, constitutes what is missing from Mark is plausible. However, students of the Fourth Gospel know that John is not a unity either. The prologue, the story of the woman caught in adultery, and the final chapter all look like additions, which means that the narrative ending we are looking for in Mark has its basis only in material which is itself mainly the phenomenon of later editorial activity, some of which was not canonized. Not firm ground to stand on.
