A little side angle.

Story:
After the black mesa incident was over, everyone thought that the alien nightmare has ended, but sadly for mankind the portals appears again, but now all around the world.
Vortigaunt race that was enslaved by nihilanth, after his defeat became free, but they needed a new home. Home that would be far away from combine empire, so they decided to invade earth once again, but now more prepared and in arms. Thanks to the armor and weaponry left by the humans in ther Xen world, they quickly learned how to craft them for them self. Then Vortigaunts opened portals to earth, but not simple portals, thoes make its user mush larger on exit.
Humanity didn't stand a chance against gigantic aliens in adapted hev suits. The only chose left for mankind is to bow down to the giant feet of Vortigaunt race and beg for their mercy, and hope that they won't exterminate them like pathetic insects.