I know Droqen's talked a bit about a desire for a sense of discovery. I think that sounds like a wonderful point to focus on! I absolutely love uncovering information, being surprised and interested, and thinking like crazy to find out or formulate theories on new things. The way I'm thinking this could work is through some sort of mystery. Something that needs to be dug into, to be dissected and investigated.
Or maybe the game could start normal, with nothing too out of the ordinary going on, with strange and engrossing things popping up and growing over time? A bit lonelygirl15-esque, I guess, if that helps (not that I've ever seen it or anything; only read about it and thought the concept and execution sounded interesting).
A point of discussion these two thoughts also raises is one of a goal - should there be a goal you're (at least initially) working toward? When I think about it, there kind of doesn't have to be. Maybe discovery is enough.
Either way, it's got to be kind of ARG-ey, right? Or maybe not? It feels like a mail-based game with a story has to be by definition, but maybe I'm missing something.