I almost typed "Ultima VII" (The Black Gate), but was immediately struck with the realisation that it's already here.
Shadowgate would be awesome, sounds like a plausible release, too - the remake got released only recently .

As for my (wild) guess...
Black & White, maybe?

Zoo Tycoon :p

Hopefully not the 2007 one by Valve.

Well, not that that would be bad, but I would actually rather like the 1986 one to get here (it has nothing whatsoever to do with the Valve game, them having the same title is most likely just a coincidence). There was a Kickstarter for a remake, but it failed miserably to get enough funding. Twice.

Could be the Gateway games based on Fredrik Pohl's novels.

Yeah, but you're too late with your guess ;) - http://www.gog.com/forum/general/this_weeks_enigmatic_hint/post2336

Whoops... well I'd still be very happy to see them arrive here.

Hmmm, I'm thinking of Maniac Mansion, but... no way, it'll never come here... :)

Iron Lord?
Yeah, BIT of a stretch that one...
Was there even a PC version thinking about it...?

Edit
Yeah there was but the Amiga version looks better
http://www.mobygames.com/game/dos/iron-lord

Gamersgate Complete Collection!
What do I win?

Free copy of Depression Quest!
Enjoy!

I would love it, so I hope you are both right :D

Let's try a more abstract interpretation:

Perimeter 2.

Is it because it's a gateway drug to more Amiga games? :D