I had this idea a while back but I never even considered trying to put it together because it would require a shitload of coding and whatnot. My idea was to play as a headcrab...yeah that's right, the lowest level of Xen evil. The strategy of being a headcrab would be to find a host body and use it to fight the humans. For example, you arrive at Black Mesa and roam around in the ducts until you find a lowly security guard. You jump on his head and use his body for combat purposes. Having to play as a "zombie" would be cool i thought. You could zombify specific humans for specific needs. If you want to pack a little more punch, try a soldier, if you need to gain access to a door or computer, zombify a scientist and hack things up. When the host body dies, seek cover in ducts and other places where you could feed on dead stuff to regain health. It would be cool to see how it worked out. Just an idea!