If we go by most desirable according to the Community Wishlist, then the results are:
1. Blizzard (Diablo 1)
2. Gearbox (Homeworld)
3. Microsoft (Freelancer)
4. Konami (Metal Gear Solid)
5. Rockstar (Max Payne)
I deem Gearbox, Microsoft and Rockstar as pretty realistic possibilities. Konami, on the other hand, doesn't seem to have any interest in developing/releasing console/PC games anymore (plus them being Japanese publishers, there would have to be some compromise from GOG's part that would convince them to join). Finally, as for Blizzard, they haven't even published their games on Steam and have their own team that make their older titles compatible with modern systems. So, while the last two aren't impossible, I doubt we'll see them anytime soon. But, you never know.
Edit: If we don't count Gearbox as a big publisher, then we can add Sony (Lemmings) to the list. I'm not sure about that. I mean, if you asked me some years ago, I would say, "Hah no way!" But then, they did publish two formerly-exclusive Playstation games on Steam under their Playstation Mobile Inc label so, again, who knows?
Post edited April 20, 2016 by Grargar