This would be extraordinarily difficult.
1. Securing the rights to use a character from (insert work here), unless they want to just stick to Sam and Max/Homestar Runner/TF2/Penny Arcade characters.
2. Making a model for that character, although Telltale has plenty of models they can reuse, like they did with Max and Strong Bad. (The Heavy is technically a reused model, although they had to modify it to work in the Telltale Tool.)
3. Getting a voice actor to record lines for that character.
4. Getting William Kasten, Matt Chapman, Gary Schwartz, and Kid Beyond all back to record lines, because this new character would be no fun if the other characters weren't having conversations with him.
5. Most likely getting Matt, Mike, and Jerry back to help with writing the new lines for Strong Bad and Tycho. (Anyone know how involved Steve Purcell and Robin Walker were with this game? Matt, Mike, and Jerry were all credited for "Additional Writing," though.)
If Telltale wants to go through the work to add even a single new character, they'll most likely just make a sequel, not an update.