Starkrun: I agree that the most likely is Homeworld
No One Lives Forever is actively being worked on by some very big fish in the old-to-you market.
And I'm going to bet that we see Fallout 3 GOTY at some point.
My Dream Wishlist is the iD games: Doom, Rage, Wolfenstein 's. and from Arkane: Dark Messiah of Might and Magic, maybe Dishonored ^_^
NOLF? Really? I thought that IP was stuck in legal hell. If Night Dive was finally able to negotiate an agreement between Warner Bros. and Fox, that's definitely news to me. Care to provide a link? :)
I have no insider information of any kind to go on, but I'm betting we'll actually be seeing the newer Bethesda RPGs coming here in the short term (6-12 mos.), in addition to the remaining gaps in the the LA back catalog like the Full Throttle remaster and the remaining Monkey Island games. With 2K Games now on GOG I'm also thinking we'll be seeing more Take Two games like Oni and the Myth series here soon, too.
I'd like to think that EA will release more of its classic back catalog like the C&C series, but I'm almost certain that they'll remain Origin-exclusive.
Homeworld would be nice, but if Gearbox ever decides to come to GOG (which I don't see happening anytime soon) you can be sure they'd be releasing their remastered edition.
On my wishlist is the last major publisher I can think of off the top of my head that isn't on GOG: Microsoft.