i) One of the generally neglected lines of evidence for the historical Jesus are the parables of Jesus. A partial exception is Keener's treatment in The Historical Jesus of the Gospels. These are mostly clustered in the Synoptic Gospels, although you have two parables (the true vine, the good shepherd), as well as many implicit parabolic metaphors, in the Fourth Gospel.
The parables are a central and distinctive feature of Jesus' teaching. Not only do they figure in his teaching, but many of his actions have a parabolic significance. Sometimes the two are tied together. He will tell a parable to illustrate an action. Or his action will be symbolic. It would be very difficult to extract the parables from the historical Jesus generally.
If, however, you deny the historicity of Jesus, then you have to account for the parables. Who wrote them? If Matthew and Luke simply got their parabolic material from Mark, it would be easier to attribute them to a single source. But Matthew and Mark have unique parables.
So an unbeliever must hypothesize an anonymous literary genius or geniuses who composed these parables, and somehow got the entire Christian community to incorporate them into the Gospels.
ii) I'd like to make one additional point: some parables indicate that Jesus could return at any moment, while other parables indicate signs which will precede his return. That's a tension that commentators struggle with. And it's cited as evidence that the Gospels are fallible.
Problem is, the prima facie tension is so obvious that it could hardly be unwitting. That tension would be discernible from the get-go.
To say that reflects a contradiction is naive, for the contrast is clearly intentional. It's something that Jesus puts out there and leaves unresolved–to keep listeners off balance. Be watchful, but not presumptuous! It strikes a balance between complacency and anxiety. A little uncertainty is a good thing; too much uncertainty is a bad thing.
This, in turn, figures in what we should make of Christ's apparent prediction that the world would end soon. That's just one side of his eschatological teaching. That needs to be counterbalanced by the other side.
If you take his teaching as a whole into account, we are kept in suspense precisely because we don't know how or when this tension will resolve itself. Kinda like a Whodunit. The novelist (or screenwriter) includes clues, not only to help the reader (or viewer) isolate the culprit, but to throw him off the scent. Early in the story, the novelist will feed the reader clues that lead the read to suspect the wrong character. To prematurely solve the mystery.
Then, as the plot progresses, that character is rules out, and attention turns to another person of interest. By process of elimination, the mystery is finally solved. And it may be a plot twist. A surprise ending. To some extent, Jesus employs the technique of a mystery fiction writer.