While Interplay is here and they've recently re-released Planescape as a retail boxed copy (something they deny completely, by the way), it's substantially more expensive than $10. So, given that they don't have the rights to re-release it (by their own admission) and they know what people are willing to pay for it, I don't see it coming here.
Add to that the fact that Wizards of the Coast don't like... anybody and are presently in a legal tangle with Atari over the D&D license and I don't see any D&D games coming to GOG. Or at least, not staying here long if they made it here at all.

It's hilarious how literally every game ever made has been speculated to be it in this thread.

Well, every good old game.

And Daikatana.

And Duke Nukem Forever!

I'm betting it's going to be System Shock 1+2. Fantastic games, immediate purchase:)
What I would really like would be a few FMV games: Wing Commander 3-4, Harvester, 11th Hour and others, or some nice later Sierra titles such as Shivers 1-2, Phantasmagoria and so on...

The very best in vaporware.

I hope for Planescape, Baldur's, Icewind...

I'm putting my money on Nethack, personally.

I see people mentioning Wizardry and that reminds me that the first time I browsed gog's catalog, I was hoping to see the series. Wizardry and Ultima series belong here.

Oh, if we get EA, doesn't that mean we could also get Clive Barker's Undying and American McGee's Alice? That would be AWESOME!

One thing is sure. It won't be System Shock 2.
As a person who's finished the game quite recently, playing on the newest mods and tools that make the game working, and browsing through the StrangeBedfellows forums, i see no way for GOG to release this game with full support. They would have to integrate it with Kolya's SS2 Tool, which still doesn't work for everyone as planned. Mostly small things like 3D environmental sound effects [EAX/A3D].
And there is no other way to launch the game on modern systems. Some of You may think of VMs, but, i assure You, i've been there. I regularly test VMs [VM Player and VirtualBox mostly] to see if some older games are playable. Sure, You can play newer titles, like Far Cry, but the pre DX8 era games? Not really. Same goes for DK, which i've mentioned earlier.
So guys, the countdown aims for something big, like a publisher, or a great game saga, like the Ultimas [yeah, which would mean EA is on GOG and SOMEONE EATS HIS SOCK as promised:P] , but it may also be something more ambitious, though unlikely. For example an experimental windows emulation platform, which would allow us to play games which might come in the future, like SS2, DK or Thief [and Wet - The Sexy Empire].
Seeing titles from EAs darkest dungeon would be great, Origin, Bullfrog, Maxis, Westwood.
It could also be something with Irrational, they're equally secretive and have some surprises in store for 2010, but that would mean a del with 2K/TakeTwo.
Too bad GOG hasn't [haven't?] decided on giving clues every 12 hours, that would be way better than cold speculations. ;)

they should make it system shock 1 and/or 2

Needless to say that if that happens, we want a picture.

Those can easily be manipulated with graphical tools, i call for a movie with 3 independent camera angles in HD. ;D