There's no way FFXIII is making it up there. The barrier is going to be over 8 million soon (higher than FFX), and rising. Honestly, Modern Warfare 2 (PS3) probably has a better shot, which is nil.
Definite new entrants next year will be Modern Warfare 2 (360), Wii Sports Resort, Pokemon Heart Gold / Soul Silver, New Super Mario Bros Wii, and Wii Fit Plus. Pokemon Platinum may creep up above 8mil later on. The barrier will only get higher as Nintendo releases more major hits for the DS / Wii, I imagine they have at least a handful left in them.
edit: Also, Mario Galaxy (Wii), CoD4 (360), and Super Mario 64 (DS) are still selling. In 2 years, the minimum may be higher than 8.4mil (Halo 2) just from the titles I've mentioned. That's definitely too high for any traditional PS3 titles we know of, besides GT. Unless Sony comes with a surprise hit, GT5 will be on its own.
