Well here's an idea:
To start out basic I'll just use the Christof remastered model that Xaltar created and have him be the main character. Instead of following the Redemption plot, I could make an alternate universe where Christof and Anezka are fighting with Gordan Freeman's resistance against the Combine. That way I don't have to do too much to start with, just learn modeling, sound scripting, and AI reaction (for Anezka as an ally). I don't know anything of how to script blood or feeding in HL2 so I'll just stick with health and soak like a normal HL2 game, but instead of using medkits to heal your health, you could pickup bloodbags (just some converted model).
The concept could be as simple as HL2: Episode One.
You are Christof trying to escape City17 before the Citadel blows. You're with your love and long time companion, Anezka, who like Alyx, can kick ass with the best of them (I could apply Alyx's AI script to an Anezka model when I figure out how.) I could add damage regions to represent sun spots for day maps sense the citadel blows up during the day, sunny maps will be plentiful. I don't think it should be hard to add the sun damage regions.
Just simple stuff. No epic plot, no Wod PnP rule coding to start with, just basic learning of the HL2 engine's script system and modeling program. See, I am not going to overwhelm myself with an overly ambitious project that has NO CHANCE OF LAUNCHING. I'm not going to code a Storyteller multiplayer mode right from the get go. I'll make a single player experience primarily. I wouldn't know where to start on a coding a coop multiplayer feature.