I think one of the things that makes the Elder Gods and Eldritch Beings of the Cthulhu Mythos interesting is the idea that you can't really get behind their motives, for the most part. The whole "Cosmic Evil" (which really is almost more of a "Cosmic Amorality", the way I most often read things) is of a scope that serves to beggar the human ability to even imagine, much less comprehend. The worshippers of these guys generally come off as pretty loathesome if not just down-right pathetic, insane wretches. The few who are down with the big bad who still have all their marbles are still working towards a goal I don't know that many folks could really feel motivated to achieve. Unless you're into crazy-elaborate and unique modes of suicide?
I think designing the game would be an interesting experiment in mechanics, and I have to admit I have a weird, morbid curiosity about (and an almost icky desire to write) the flavor text for all the cards, but I don't think, in the end, it'd be a game I'd want to play.
I kinda really groove on trying to save the world. It's where I keep all my cookies!