I would like to get:
Final Fantasy VII and VIII
Metal Gear Solid
Baldur's Gate
Icewind Dale
Duke Nukem 3D
Warhammer: Dark Omen
Mortal Kombat Trilogy
... and hope it is possible to see these games in the future!!

Hmmm, just off the top of my head...
Jagged Alliance 1 & 2 (never played, but would love to)
Planescape: Torment
Betrayal at Krondor
Crusader : No Remorse
Crusader: No Regret
Master of Magic
Master of Orion 1 & 2

All the Mortal Kombat games that have been released on PC(i.e, MK1, MK2, MK3, MK Trilogy, MK4)

oni

Sid Meier's Civilization II

Yeah that would be great.

System Shock 2 (since I can't get it to run under vista).

My wife would be ecstatic if GOG could get her a functional version of Constructor. She loved that silly thing...

I would be so happy if we could manage to get Mechwarrior 2 (and expansion!) on the list. I don't know what it was but every version I had of that game glitched out at the most random parts. I never knew if it was the game or disk I had.. it made me sad.
Echoing those saying System Shock series and Deus Ex

+1, didn't have a pc that could run it well at the time.

Blood & Magic
Battle Isle 2 + Titan's Legacy Exp.
Destruction Derby 2
Settlers II Gold
Warhammer - Dark Omen
Z + Expansion Kit

Fable
Well, okay, it's not really all that old, but I can't find it in any other online digital distribution site...
And I saw a racing game from 2005 that's going to be released, so... yeah.
And Pandora Directive.

MechCommander Gold
Mechwarrior 3
Elite Plus
Frontier Elite 2
Wing Commander
Thief 2 The Metal Age

MechCommander series is freeware. Go to http://www.mechcommander.org/ for downloads.