Here's who I think would be a good fit:
The Big Name: Ryan Reynolds
Reynolds has made a name for himself as the sexy funny guy with a gift for comic timing. He's currently busy juggling two comic book franchises (Green Lantern and Deadpool), but if the studio could snag him to fill Chase's shoes, I'm sure they would.
The Physical Humor Expert: Zachary Levi
If you watch "Chuck," you know that Levi is a pro when it comes to pratfalls, stumbling, and generally making an ass out of himself. Fletch, however, is also clever with words, and Chuck doesn't really get the best lines, so I don't know if he could pull off cocksure quips.
The Facial Expression Dynamo: John Krasinski
Krasinski is a bit of a scene stealer. Whether it's a bad movie (License to Wed), a little-seen indie (Leatherheads), a romantic dramedy (Away We Go), or a star-studded comedy (It's Complicated), he has a way of stealing the spotlight with just one look or verbal stumble. So I'd like to see if he could pull off playing a know-it-all.
The Reluctant Comedian: Shia Labeouf
Labeouf hasn't done a straight comedy since his bit role in 2003's Dumb and Dumberer. And he hasn't played the comic relief in a non-comedy since 2005's Constantine, unless you count the Transformers trilogy, which comes to an end this summer. These days he seems more focused on dramas (Wall Street: Money Never Sleeps), action films (Indiana Jones and the Kingdom of the Crystal Skull), and crime thrillers (The Wettest County in the World). So wrangling him for a comedy about a rule-breaking investigative journalist with a wacky way of digging up the truth might be a little bit of a hard sell. It might also be a hard sell to audiences who haven't seen Labeouf's humorous side in almost a decade. But I really do think he has what it takes to skirt the line between suave and hilarious. Come on back Shia!