The Ghost of Christmas Future is shrouded in mystery, all at once looking like a shapeless, faceless form in a black robe, while at the same time, it looks like the Grim Reaper or death.
Symbolically, the spirit represents the unknown, the future which is not written. However, it is also symbolic as death which is what Mr. Scrooge has to confront in this part of the story in order to earn redemption for his soul and change his ways.
Dickens keeps the character undefined, because it depends on Scrooge's willingness to learn the valuable lesson that Jacob Marley wants him to understand in order to change his fate. If he decides not to change, the future fulfilled will reflect the hollow, empty life he now has along with an uneventful death that no one will mourn.
The interesting thing about the ghost of Christmas future, or the "Ghost of Christmas Yet to Come", is that Dickens refers to it as a phantom rather than a ghost like he did with the other apparitions.
This phantom was perhaps named as such because of how different it was from the others. While the first ghosts interacted and showed some leftover traits of humanity, this one was eerie, silent and quite scary, not only in movement but also in description
It was shrouded in a deep black garment, which concealed its head, its face, its form, and left nothing of it visible save one outstretched hand
The way it moved was just as sinister as it looked
SLOWLY, gravely, silently
The phantom never answers with words, and allows Ebenezer to make his own conjectures every time he asks a question. That psychological dynamic causes more fear and anxiety in Ebenezer than having the actual answers told to him. Moreover, once the finger of the ghost points at something, it does not back down until Scrooge actually looks at what he is pointing.
While he is the phantom Scrooge is more scared of, his ending is quite abrupt and coarse for a ghost so apparently sophisticated.
he saw an alteration in the Phantom's hood and dress. It shrunk, collapsed, and dwindled down into a bedpost.
Therefore the ghost starts out extremely sinister and ends quite coarsely to a point. However, this is undoubtedly the ghost that influenced Scrooge the most, emotionally.
Just a quick response to Michelle Ossa's post. I don't really agree that the Ghost of Christmas Future is the one that affects Scrooge the most emotionally. Sure, it terrifies him, but it's his transformation when re-visiting his childhood and youth, under the influence of the Ghost of Christmas Past, that is his most intense and life-changing experience. This is what sets him on the road to redemption: the re-discovery of old feelings and joys which had been 'long, long forgotten'. It is like the sudden bursting of a dam of pent-up emotions, which I think is the most poignant aspect of the entire story. It is important to note that he becomes increasingly open and light-hearted from this moment on; he delights in the various displays of Christmas happiness that the Ghost of Christmas Present presents to him, and shows genuine concern for Tiny Tim, and so on.