I think the most likely surprise guest might be Mark Hamill. He does all of that voiceover work, and I think he's based out of LA. I'm personally hopeful that it might be Frank Oz, but I don't think he ever does this sort of thing.
But even for a "seasoned" autograph collector like myself the costs are starting to stack up for C4 autographs. My personal estimate on what I'm getting is bordering on $500+.