So a text based manga game with a command line style?
Sounds pretty unique. Personally I'd go for a 'ghost in machine' plot - maybe you discover some AI 'being' who has gone rogue, or developed itself and is trying to prevent some apocalyptic event, or government conspiracy.
Maybe yourself and other players are hackers, sharing information to access top secret data, banks, defense installations, etc etc. Perhaps being able to make fake viruses and worms would be cool - like a little script language or abstracted assembly language system, so player compete to crack each others computers, as well as other systems.
I'm sure you know the sort of thing I'm talking about, the start of the first Matrix movie, or Wargames - I think the biggest challenge would be how to maintain real depth to the game, and the only way to really do that is to let it expand through the end user.
I am the one who knocks...