I love questions like this so it got me thinking a bit more and initially I thought specifically within the metal realm. Neil's talents certainly dont begin or end with metal, so here are some of my other thoughts.
Mainstream artists:
Coldplay
Maroon 5
Christina Aguilera
Tool
Audioslave
Dave Mathews
Marylin Manson

Eminem
Then I was thinking about assembling some specific players for a one off project. And as I was trying to put together a jazz group of sorts, it occurred to me...why not just re-assemble any willing members of the Mahavishnu Orchestra? What fun!
And while we're at it, is their any magic left in some of the once great, aging rockers still trying to be relevant?
Aerosmith? LOVE their 70's catalogue. Do they have another Toys/Rocks left in them?
Paul McCartney? I LOVE a lot of his solo stuff but the last one I liked was Flaming Pie. Does he have another 'classic' left?
Simon and Garfunkle? I thought they were getting back together, at least for a couple tunes.
Genesis? Can Neil get these guys back together and put Phil Collins back on the drums with Peter Gabriel on vocals again? That'd be cool...
I guess I could go on and on, but what a fun question. Would there be any interest in these bands if the opportunity came Neil? Surely there are artsists that YOU want to work with but just havent had the chance...
Peace
Joel