To continue this discussion officially from this journal.
It's beginning to look like a mod or map pack that is based around finding, killing all of, or collecting whatever within a detailed and exploratory environment.
Some of the ideas which arose so far are:
Headcrab Hunt
Rat Hunt
Zombie Hunt
Collect the Keycards or "Red Coins"
Probably more if Source gets involved.
Whether Spirit of HL needs to get involved is a matter of how far you want to take the idea. Personally I don't see a need for it unless someone really makes a breakthrough map that requires it.
I'm going to attempt to make some kind of rat-grenade map because that sounds ludicrous and fun. Don't let me stop you if you want to do the same thing. I also like the idea of collecting "Red Coins" and having to scavenge for 20 of those in a map.
Continue thinking, this will only fail if we give up on it.