In retrospect, Bumblefoot seems like the obvious choice. MP has been jamming with him on and off for the last several years in various projects. This definitely does add a metal element to the project. Kind of surprised with two strong songwriters like Bumblefoot and Derek that they would let Mike have total creative control. Bumblefoot actually peaks my interest but it immediately wanes when I think that he won't be a full partner so to speak.
BTW, Bumblefoot is an interesting character. He only actually recorded a few things on Chinese Democracy from what I know. Interestingly, they kept most of Buckethead's solos even though Bumblefoot replaced him. Bumblefoot did actually record solos on a few things, though. Most notably on songs Buckethead wrote.
I have one of his solo albums. He shreds but it is almost more punk than metal. Haven't heard anything else.
Oh, and he rocks at the fretless guitar.