Kermit would be the easiest to do from scratch. I actually got the idea in a round about way.
I was thinking of the avatar used by AnimalK. I thought it would be funny to have a Muppet in a kilt who's full mask Sporran talked too. Then I thought, why not make a sporran out of a muppet? Animal would be a good one, so would Rowlf. I'm not a big fan of Elmo anyway so I did a mock up of him 1st.