My question is what does the ghost of Christmas yet to come look like in chapter 6

The Ghost of Christmas future is a mystery and he looks the part. Dickens refers to it as a phantom rather than a ghost like he did with the other apparitions. He has a very Grim Reaper look to him,

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.