I don't think a "story mode" will ever be officially added.  It might be an option in SINS 2, but as of today there has been no indication that one is forthcoming.  However, you can still play it singleplay.  I do, exclusively, and it's a lot of fun.  Well worth the sticker price.  Also, the modding community is always doing something new and interesting that gives the game a new spin.  I'd recommend going ahead and getting it if you like 4X and or RTS style games.  This one has it's foot in both doors.