Actually there are already way more than 500 games in the list. A quick check through the list tells me more than 550 – not counting the many expansion sets. But ok, “500 buyable game packages” does not sound so thrilling. ;-)
My guess is that the 500th game is no single one, but a whole set of three or more games. Most interesting would be some games from Lucas Arts. Or it could be Command & Conquer. My third guess was Civilization, but as there are already three strategy games this week, this is quite unlikely … hm. Also not Warcraft or anything else from Blizzard.
I don’t know. Something action-related. Surprise me, GOG!