All of the game listed below have either already released a Linux port, announced that a Linux port is being developed or talks of a Linux port is in the works. All updated information on Linux releases are from GOL and the SteamDB website.
Age of Wonders III Baldur's Gate: Enhanced Edition Borderlands 2 Borderlands: The Pre-sequel Brutal Legend Company of Heroes 2 Darksiders Darksiders 2 DayZ Dead Island Dungeon Defenders Eternity Empire: Total War Firewatch Half-Life 2 Homefront: The Revolution Icewind Dale: Enhanced Edition Left 4 Dead 2 Metro 2033 Redux Metro: Last Light Redux Mount & Blade: Warband Portal Portal 2 POSTAL 2 Project CARS Psychonauts Serious Sam 3: BFE Shadowrun: Dragonfall - Director's Cut Sid Meier's Civilization V Sid Meier's Civilization: Beyond Earth Team Fortress 2 Torchlight Torchlight II Total War: ROME 2 Unreal Tournament (new one) The Vanishing of Ethan Carter The Witcher 2: Assassins of Kings Enhanced Edition The Witcher 3: Wild Hunt XCOM: Enemy Unknown
It's more important to worry about SteamOS's future rather than adding existing games.
The way I see it is, Steam Machines are supposed to compete with consoles. Neither the Playstation or Xbox have backwards compatibility, so older games are completely irrelevant for that market anyway. The thing that really matters is what kind of support will Steam Machines have at release. Will the next GTA support it? What about CoD? Dark Souls 3? The next Tomb Raider sequel? If these kinds of high-profile mainstream games actually do have flawless day-one SteamOS/Linux support then I can see it being a damn compelling platform, regardless of whether or not it'll run games from the past.