Apparently some people theorize about the Combine having control over Xen. How would that work though? The G-Man's employers had control of Xen at the end of HL1, and if the G-Man was a representative of the Combine, he would just kill Freeman. On top of that, if the Combine had Xen, they would have developed local space teleportation technology from the portals just strewn all over Xen and the technology that Black Mesa's survey teams brought to Xen before the resonance cascade. They also would have probably converted at least one or two of the intelligent species inhabiting Xen (Eg, Alien Controllers, Alien Grunts) to be included in their arsenal of weaponized species.