Personally I cant see Subaru using any other car other than the Impreza in any form of motor sport without shooting themselves in the foot.
They MAY be able to get away with using a Legacy, but the Impreza name is sin-ominous with rallying (and other motor sport) so to use another model would leave people asking what the point of the WRX STi was anymore.
As for the Sub-ota (SUbaru/Toyota joint venture) anyone know what car specs its likely to have? pics?