Treadstone1-5: Anyone got any ideas which games you believe GOG will never be able to bring back?
My money is on:
No One Lives Forever, Simpsons Hit and Run, Mafia 2, Escape from Monkey Island
The NOLF games are well known to basically be held back by legal issues. Legal issues can get sorted out over time by the people involved actually caring enough to want to resolve them on all sides and coming together to find a mutually agreeable solution, so I wouldn't come close to saying that seeing NOLF show up here on GOG in the future is impossible. It's entirely possible.
I do not know anything about the other games mentioned and what their official status/story is, however the bottom line about any game coming here is whether the intellectual property owners all combined have a financial interest in bringing a game back from the dead and are willing to put some priority on resolving any legal issues, technical issues or other in order to make it happen. As such it is theoretically possible to see just about any game ever made show up on GOG if it fits into a category of things that GOG actually sells and meets their other criterion or if GOG's criterion changes over time to embrace other games that do not presently qualify for potential sale here.
In essence, everything is malleable. Some games are more likely to show up here than others and some are much less likely but the likelihood of any particular title showing up here is never 0% or 100% - there is always a possibility in the future of anything showing up here. Despite how unlikely it seems that a particular game might show up here, we have already first-hand seen quite a large number of games show up here in the GOG catalogue that prior to their release many people would have sworn up and down, left and right that they'd never ever come here in a million years. And yet - those very games are sold by GOG right now.
What is the lesson to be learned? Never say never, because we simply do not conclusively know that nor have a way to know that. All we can ever know conclusively is "not yet". People who own crystal balls excluded. Not because they can see the future mind you, but because they're batshit crazy. :)