I notice that you have some prog on your list.
Might I suggest:
1) King Crimson, as well as related projects such as Robert Fripp, Fripp/Summers (yes, Andy Summers), California Guitar Trio (completely instrumental guitar music), Tony Geballe, League of Crafty Guitarists. Of note, the album "The Compleat King Crimson" covers 20 years of the band's production, and it's version of "Sleepless" appears nowhere else that I know of. The band's post 1980 work is considerably harder edged- the 80's stuff is comparable to the hardest stuff Yes ever did, the stuff from the 90's and later has some real edge, but is still much more prog-rock than metal.
Artists who are/were members of King Crimson or worked with Robert Fripp: Adrian Belew- solo, Bears, Talking Heads and many more; Greg Lake of Emerson, Lake and Palmer; Tony Levin; Bill Bruford; Trey Gunn and others too numerous to mention.
Also of note would be such artists as David Bowie's guitarist, Reeves Gabrels (with or without Bowie/Tin Machine), Brian Eno, David Sylvan, and the great bassist, Jonas Hellborg (he plays pop, jazz, fusion, metal, etc.).
2) Shredders:
Jennifer Batten, Ronnie Montrose, Joe Satriani, Steve Vai, Alex Skolnick- for your purposes, stick to his jazz/metal fusion stuff as the A.S. Trio or Trans-Siberian Orchestra, as opposed to his work with Savatage or Testament (despite being of the same genre as Metallica, Chuck Billy's vox for Testament are a bit more growly, though not like the modern grunters).
Other shredders of note- Shawn Lane (the fastest guitarist of all time, and tasteful jazz/metal fusion artist, R.I.P.), and Al DiMeola (solo or with Return To Forever or Miles Davis).
3) Off the beaten path:
Rodrigo y Gabriella- a Mexican guitar duo who rip up Metalica on accoustics, among other things.
Seu Jorge- South American guitarist who was a member of Farofa Carioca, but most famously appeared in "The Life Aquatic with Steve Zissou" as a crewman who played accoustic Portugese versions of David Bowie tunes.
Kronos Quartet- a chamber music group who play all kinds of modern composers from all over the world.
Buckethead- a shredder's shredder, but more than that, too. Careful though- his stuff runs from utter shred to trancelike techno, so you might want to listen to samples of a particular album before you buy it.
Glen Branca- avant garde composer who uses rock instrumentation to do modern metal-classical fusion. This isn't "symphonic metal," this is modern symphony using things like electric guitars in non-standard tuning, multiple drummers and percussionists, unique instruments created for the piece in question, etc.