John Mayer
This. Guy. Can. Flat-out. Play.
Nuno Bettencourt
Yeah, he did Generation Axe already, but would probably do well in G3.
Vito Bratta
A lot of 80s guitarists could really play, it's just that the music was "easy" and they dumbed-it-down for the songs but give them a 10-minute solo spot in their shows and they can walk the walk for sure. I don't think he's making music or playing at all, though.
Polyphia and Arch Echo would be nice but probably wouldn't really fit-in unless they were an opening act or something cuz nobody is "famous" in either band.
Guitars that sound/look good on paper but probably wouldn't work out that well:
Slash
Not really a solo performer, needs a full band with vocals to play with, other than jamming.
Mark Tremonti
Too much wah-wah, and also needs a band, same with Kirk Hammet.
Richie Sambora
15-20 years ago, for sure, nowadays, aw hell no, but Orianthi would be cool.
Eddie Van Halen
30 years ago, for sure, nowadays, aw hell no.