I believe Multiplayer would be interesting if the Humans had to "escape" from a building of some sort, and the headcrabs (Also players) spawned indefinatly. What I mean is, when a headcrab dies, it respawns, and when a human dies, hes dead for the round. Get it?
also, Clever level design in favor of the headcrabs would be very intersting, such as a metal complex with tons of crevaces, holes, airvents, and such leading right to where the humans must get through.
Also, tight passageways and such would be great.
If this is the idea you are going for, I would love to map multiplayer for this later on
(I just gotta learn more about HL2)
Tell me if you need a mapper for that sort of a map
I would also recommend NO USE OF ZOMBIES. The headcrabs alone would suffice.
(Although, it would be cool if a headcrab kills a human, and stays on him long enough, he turns into a zombie, and then after time, he could become the next stage of zombie, freaky Jumpy skeleton dude.)