What game would you make it a mod for? Just as a heads up, I'm not actually interested in applying the results of this poll - my FPS modding skills are pretty much zilch. Nor do I want to use it as a suggestion to Sandwich anybody else thinking of this project, as I know Sandwich is set on Serious Sam, and anybody else who might take up the project would probably only do it if they already had an engine in mind. I'm simply curious.
Anyway, my vote would go to Half Life. I know the engine's dated, and it doesn't have item use, but it does have several big pros, eg Availability (Everyone's got Half Life, and it's still being sold), Low system specs (Leading to a wide mod dissemination) and a massive community with tonnes of modding experience, and therefore presumably a fair number of tutorials, tools and helpful forumites.
Any other nominations?