RetroJaro: The trouble companies I can see are blizzard, maxis, id software, apogee, and epic megagames.
Sim City 2000 is already in the catalogue. I think the odds are rather good that we'll get more Maxis games sooner or later along with some other EA games that are still missing. Heck, I even honestly believe that The Sims Complete has a decent chance to make an appearance on GOG.
Among my "favourite games that will never appear on GOG" are The Punisher and Marvel Ultimate Alliance. The Punisher may be trashy compared to say Max Payne but IMHO it's still an awesome game, especially when you're a fan of the comics and actually play for score (which turns it into a surprisingly tactical experience). Haven't played Marvel Ultimate Alliance myself yet but from what I've heard it's pretty darn awesome. But with Marvel belonging to Disney... who knows. Then again, I still haven't given up on LucasArts games so if GOG manages convince the mouse to release those ones here there might also be a chance for some games based on a Marvel license.