When ever there are multiple expansions for a game, the $20,000 question always is, are they all needed?

For example, once Twilight is released, will a new player have to purchase DA in order to get all of the features in Twilight? If they do not have it, will they only have access to certain features? Or, will the only difference mean that they won't have access to the campaign missions for DA (which I don't think really makes $29.99 worth it to buy DA once Twilight is out)?

Basically, once Twilight is released, will DA really be necessary for someone to buy, or will all of the "keeper" features just be rolled into Twilight?