I've just noticed something. If Chapter 21 were a late addition by John or his friends to the Gospel, then one might expect that the end of Chapter 20 would be a more stylistically and thematically planned ending to the Gospel. One might expect it to match the Prologue, in fact.
The "light of men" comes on the First day of the week when light was created; the doors are shut so that the world, who did not receive him, remain in the uncomprehending darkness; he is the Maker of all things who is present on the Eighth day as well as the First, emphasising that everything in between was made by him also; on the Eighth day his bodily presence is shown to Thomas, paralleling the "Word made flesh"; Thomas (his "own") did not receive him; he gives power to his own to become the sons of God, and empowers them to make others God's children by breathing his Spirit upon them (not by the will of man or the flesh, but by the will of God); the John of the Prologue is paralleled by the witness of John the Apostle in the Finale; and the motif of faith in his Name is present in both sections. The ordering is not identical: but I think there is a case to be made that this is the original ending. I am sure that this has been noted before by somebody, somewhere.
I also think that this parallel (on account of the naming of "John" in the first section without the qualifying "Baptist") gives further weight - if any further were needed - to the authorship of the Apostle. The name of John in the Prologue is a cryptic clue to the identity of the author, when taken with its parallel final section: he is the witness sent from God.