Game sharing was already announced. Steam already has a mobile and app version, and a fully-fledged Steam phone sounds kind of dumb. Game announcements wouldn't really make sense because this whole thing is about bringing Steam to the living room. Also, many people would be pissed just at the thought of grouping Valve games/game engines and "Steam in the living room" together. I think Valve knows that, so that's another reason why I don't think it will be about a game or game engine.
I'm stumped for what it could be, though. Someone mentioned a controller earlier, but that would make for a pretty weak final announcement. The controller theory was drawn from the Steam Machines announcement, so making one announcement about something that already played a minor part in a previous announcement would be a bad idea.
It could be like a community overhaul or something. Both the Xbone and PS4 highlighted socializing and streaming as part of their new features.
Controller. Why would they make their own OS, to run on their own system, and then use a 360 controller? And it'll be something with build in mouse emulation/programmable buttons for any keyboard mapping so that it universally works (to a degree based on the game obviously)
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