There are two main routes that this could take:
1. Sony's motion controller becomes compatible with the Wii (somewhat) allowing more games to be co-developed between the PS3 and Wii. This singles out the 360 and Natal, isolating its audience and lowering its developer support.
2. Natal takes off, while the Arc fizzles out. Microsoft integrates Natal into the Xbox Live community games (such as Uno), and simplifies the control scheme; all while implementing the device in several big name games. The 360's gap over the PS3 widens, and the Xbox brand gains an entirely unique and popular control scheme.
Number 2 is more likely, but I'd rather see #1 happen (mainly because it's pretty much the closest we'll get this gen to a Wii HD, with big HD games implementing motion controls).
