Flood because there is a 5/1 ratio. Also cuzz when the flood take over corpses they ratain knowledge unlike headcrab counterparts (so they could use their guns, and they are more agile. Even been up high and get pounced? I know I have)
My dream gun is a .65cal kevlar tipped, depleted uranium, high velocity cartiges. With the trigun revolver but with extended barrel about 1 1/2 inch and a 10x top mounted scope /w night vision and heat vision.
Rift in space time, makes a portal from one dimension to another. Like what John Titor used to get here. Be he used black holes instead of a xen crystal and a spectrometer.
At the end of hl1 G-man says something like, "Thanks to you we have captured the border world of xen, nasty piece of work you managed." Those Alien salves could be clones or infact alien salves of the humans
What if instead of us saving the world we are accualty destroying it. See in HL 1 G-man manipulates us to destroy xen. Now that we are done with that, years later the combine have come to make order in the world. All xen aliens have been attacking and what not so the UN makes the combine to...