The SteamDB group have unearthed something fairly interesting in the coding for Steam's latest big Beta update. Seems that Valve are already beginning to build in places for apps for game play things such as demos and DLC as well as other types of media such as music and TV.
Valve had already revealed the news of Steam Music earlier on, with a beta client for the new app being sent to testers in February. There has also been film released via Steam already, the Dota 2 documentary Free-To-Play as well as award-winning film Indie Game: The Movie although both are still stored into the game library at present.
This capability to fit apps into Steam's architecture suggests that as well as trying to conquer the traditional part of the console market with their Steam Box console and Steam Controller (now delayed until 2015) Valve might be planning to offer a complete package of media to rival that of the Xbox's upon the Steam Box's launch.
It'll be interesting to see how it turns out.