Why not just get a PC?
Ask the thousands of people who are clever enough to use XBMC to create their tv system, and the millions more who aren't clever enough/can't be fecked, but want exactly that.
It's just a PC weaste. You can upgrade the RAM, Video Card, CPU, Network Card, Bluetooth adapter, anything. Just like a PC. You can run Windows on it, just like a PC. Will valve subsidise it? I'm not sure. Will it offer better value than the PS4/XBOne? I'm not sure. Maybe. Cheaper games, bigger library, you never lose the games, upgradeable, etc. If it has access to everything else you want in a media centre, I think it will do superbly.
Split screen will be important for me though, as will the question of can others use the games you have bought on that device. Lets answer these questions first.