BentBarrel,
As I understand it, the main rule in bunjin is that there are no rules.
But that doesn't mean that any old tree is bunjin. There must be artistry to it.
I have only created one bunjin so far. It hasn't much in the way of taper, but the base is interesting due to some exposed roots and considerable scaring on the lower trunk. I've positioned it to show off these features, which lead up to a smooth, twisting stretch of trunk and finally a burst of leaves at the end, pointing out toward the viewer. (I need to take a photo of it; don't have one at the moment.)
The tree is a green ash, which ordinarily wouldn't work well for bonsai because it grows rather large compound leaves. But right now the leaves are still small and it works beautifully. When they get larger, well, it's bunjin, so I just throw the rule book out the window.