See those human actors walking around in Victorian-era clothes in the crowd scenes and singing in chorus? They're not just extras. Most of them are the puppeteers. Look closely and you can just about see the muppets some of them are walking with. It gives the potential for even more Muppets to be merged with the population of Dickensian London.
The Ghost of Christmas Past is the only one with a defined form. That is because the past cannot be changed and will always be the same. The Ghost of Christmas Present ages because the present is ongoing and constantly changing. The Ghost of Christmas Yet To Come has no face because the future can't be known and is scary.
Even then, notice that the Ghost of Christmas Past is kind of blurry and ill-defined. That's because memories are fuzzy and not always perfect.
Why does omniscient Gonzo/Charles Dickens fail to catch Rizzo when he jumps from Scrooge's gate? Because Rizzo is not part of the story and thus Gonzo doesn't actually know where he'll land. Also Rule of Funny.
The Ghost Of Christmas Present tells Scrooge of Christmas Yet To Come, "Go forth, and know him better man!" But why would you want to? He's so creepy! No, wait... once you know the shape of the future a little better, it's not so scary. You can change it and make it better!
On first viewing, Tiny Tim's Incurable Cough of Death might just seem like clichéd shorthand for "seriously ill." But a little research reveals that it's actually realistic. Some Dickens scholars believe that Tim's disease is supposed to be tuberculosis, which can make the victim develop a limp by spreading from the lungs into the spine and bones. Dickens himself had a nephew who was crippled by TB and eventually died of it, and that nephew might have inspired the character of Tiny Tim. (Another notable, fictional TB cripple: Ratso Rizzo in Midnight Cowboy, whom the Muppets' own Rizzo the Rat is named after.) Tim's coughing in this movie is actually a case of Shown Their Work!