Maybe Zidane was suppose to take Garland's place as well as taking Kuja's. And Garland's place was ultimately to make sure that Terra assimilates with Gaia because all of Terra's natural resources had all been diminished. So maybe Garland intended Zidane to take up his job after Terra succesfully assimilated with Gaia, just in case Terra would have to assimilate with another planet in the future, Zidane would be the one that would make sure the flow of souls went smoothly.