David Mamet!!! how funny. I can just hear the the dialogue surrounding “mmmm…donuts” and “eat my shorts”
Jon Stewart and Co.
Rob Zombie for a Treehouse ep.
Jim Jarmusch
I don’t know how funny it would be, but I’d love to see what Harlan Ellison would come up with.
Alan Moore
Quentin Tarantino
Trey Parker and Matt Stone
Jack Black and Kyle Gass
And I agree with the aforementioned Joss Whedon, Seth McFarlane, Charlie Kaufman, Mitch Hurwitz, and David Cross and Bob Odenkirk.
Steve Gerber
Gore Vidal
Harlan Ellison
Rowan Atkinson and Ben Curtis
Stephen King
Dean Koontz
Kafka!
Fine! If we’re gonna go all literaturey, then Nabokov! 
That’s who I came here to mention. No matter what he did, it’d be a blast.
Christopher Moore
Toshio Maeda.
“Cry HAVOC!”
Annie Proulx. She did this thing once about gay cowboys… hilarious!
If Stephen King and Andrew Lloyd Webber ever collaborate on a Simpsons episode, a lot of people I know will be having multiple orgasms.
Kinky Friedman
Clive Barker
Hunter Thompson
Joe R. Landsdale
Paul Dini.
The first two that came to mind are:
Joel and Ethan Coen
Quentin Tarantino
“I Have No Chin, and I Must Scream”
Zach Braff
Robert Anton Wilson
Flann O’Brien
Richard Harris (yeah the actor, hey why the hell not, the guy is a hoot)
Emo Phillips
Gene Wilder and Mel Brooks (C’mon, Young Frankenstein was freakin’ hilarious)
Lloyd Kaufman and Michael Herz for a “Treehouse” episode
Dave Chappeale.
I know I jacked up the spelling, hey, its early for me.
Larry David
Dave Barry
David Sedaris, definitely.