Valve and Activision are the only current companies that could pull it off. Theoretically, a Valve console would be completely integrated with Steam and would probably serve as more of an alternative to a dedicated PC than its own thing. They'd have to tread carefully to make sure that it'd be profitable, but they probably wouldn't need to worry about support from gamers as a whole. This would impact Sony more than anyone else, but probably not enough to do serious harm. If Sony's smart, they'll try to tighten their alliance with Valve for their next system to benefit both of them.
Activision is capable of doing it mostly because they've got so many cash cow franchises that they could make exclusive to their own system, with World of Warcraft being an always-present option as well. This would seriously damage Microsoft, though.
I don't think the 'big three' will ever grow to a 'big four', though. Sony's in a pretty precarious position at the moment, and unless they act with tact, I fear for the future of their consoles. And if they vanish, we'll probably see a sort of East-vs-West thing between Nintendo and Microsoft, with Valve, EA, and friends chillin' on the PC to overlook it.