All right. Look. People. This is not how Muppet adaptations work.
Well, okay, sometimes it is, but that's under specific circumstances (see below).
There are exactly two kind of Muppet adaptations: Movies like Muppet Treasure Island, Muppet Christmas Carol, Muppet Wizard of Oz, etc., and sketches on The Muppet Show. One of these is taken entirely more seriously than the other, and more importantly, the casting is different.
In a MTI/MCC/MWO-style adaptation, always, the following characters are played by humans:
Any relatives of the protagonist.
One character extremely relevant to the character's plot arc.
In Muppet Treasure Island, Jim Hawkins was human; so was his mother; so was Long John Silver, who may have turned out to be the bad guy but was at least pretending to be Jim's friend, as well as a handful of background pirates. In Muppet Christmas Carol, Scrooge was human; so was his nephew (and all his nephew's family); so was Belle, the woman he loved but lost to his miserliness. In Muppet Wizard of Oz, Dorothy was human; so were Auntie Em and Uncle Henry; so was the eponymous Wizard of Oz, which makes sense both from a "he's crucial to her journey" standpoint AND a "he also came from outside Oz/the same part of the US as her so it stands to reason that they're both distinct from the residents of Oz" standpoint.
In a Muppets Lord of the Rings adaptation in this style, Frodo would be human. So would Bilbo. So would one other character crucial to Frodo's journey, which I would argue in this case would be Gandalf. (As a bit of a meta joke, I would also argue that Gandalf would in this case be played by Brian Henson.) You could also make an argument that Merry, Pippin, and Sam would be human as well - Merry and Pippin are both related to Frodo, while Sam isn't but his daughter does eventually marry Pippin's son - which would actually make sense, in the same way that the Wizard of Oz being human makes sense, because Hobbits are so strange and unusual outside the Shire that nobody's heard of them, so naturally all the other denizens of Middle Earth are Muppets.
However, if this is a Muppet Show sketch, the single human is going to be whoever the special guest is that week, which means you'll have one of three options:
If the guest star is an actor who was in the original LOTR films, they will be playing the character they played in those films.
If the guest star is someone entirely different, they will be playing whatever character gives them the best opportunity to play against their favorite Muppet.
If the LOTR sketch is a recurring sketch a la Veterinarian's Hospital or Pigs In Space, the guest star would be a character who isn't usually in the sketches/is only in that one scene.
So under these circumstances:
In Scenario 1: It's a one-off bit of the scene with the Council of Elrond. The special guest is Hugo Weaving struggling to keep a straight face as the Muppets argue over what to do with the Ring. Crazy Harry ends the sketch by blowing the Ring (and probably the entire Council Grove) up.
In Scenario 2: It's either a scene between Aragorn and Arwen with Miss Piggy playing Arwen and the special guest playing Aragorn and trying to be serious and romantic while she overacts like hell, or it's the battle against the Witch King with Sweetums as the Witch King wherein he declares "No Muppet can kill me!", leading to the special guest (who's been fighting like a Muppet this whole time) dramatically whipping off their helmet and yelling "I AM NO MUPPET!!!"
In Scenario 3: The special guest is Ian McKellan and he's playing Treebeard.
