The Xindi Empire was a potential future Xindi government which the Xindi-Reptilians might have someday led. It was referred to from the perspective of the 22nd century. (ENT: "The Council", "Zero Hour")
The notion of a Xindi empire that would be truly united, and stronger than the Xindi's 22nd-century council-led alliance, was referred to the Xindi by the Sphere-Builders (who were otherwise known to the Xindi as "the Guardians"). The concept was referenced by a female Sphere-Builder who was trying to persuade the Reptilians to betray most of the other Xindi species by launching the Xindi weapon against Earth without the authority of the Xindi Council; the purportedly foreseeable empire was said to eventually result from the Reptilians and Xindi-Insectoids nonetheless deploying the weapon.
Furthermore, the same Sphere-Builder told Reptilian Commander Dolim, since she was holographically appearing on the bridge of his Xindi-Reptilian warship, that the Sphere-Builders would guarantee Reptilian rulership of the Xindi Empire if Humanity was eliminated. (ENT: "The Council") Fully convinced by the Sphere-Builder, Dolim later relayed the idea to some of the officers under his command, while they were alone together. (ENT: "Zero Hour")