Neither DO, SD, nor the upcoming vampire GP merely expand on existing gameplay though. For me EPs usually add a larger overall"theme" to your sim's lives by adding a lot of small less specific gameplay (but with a somewhat wider appeal if that makes sense) & a world. It's more about new modes of living that can appeal to a wide variety of sims. GPS add only a single smaller theme, but really go in depth on that theme, often adding fundamentally new gameplay mechanics and fleshing these out as much as possible. So basically yes, EPs expand the world and overall variety in gameplay, while GPs deepen a very particular aspect of gameplay (hence game pack :D). I'm tired but it's something like that. That's why I think Generations would fit the GP theme really well for example: because it would expand on family game play but wouldn't have to add a new large world and what it adds would be very specific.
