the.kuribo: The missing Sierra adventure classics (Colonel's Bequest/Laura Bow series, Black Cauldron, Codename: ICEMAN, Conquests of Camelot/Robin Hood), Jones in the Fast Lane, and those Les Manley adventure games by Accolade. Oh, and the multitude of MAXIS sim games, like simant, simfarm, simgolf, etc.
I just don't think demand is high enough to justify chasing after the rights of these older titles, and I do not envision demand ever increasing at this point.
Also, games which for one reason or another GOG does not feel fit their catalog (educational/training, sports games, adult games and other niche-genre games).
simgolf is what I really want to see on here but It don't look like its happening