As far as I can tell, the opening up of the campaign is to be done with the help of community members and as such given as a patch/free upgrade, while an add-on is most likely also in the works. You shouldn't be missing anything by buying it now, unlike was the case with ADF/TAW (at a point Steve said something to this effect as well). I'm not sure if the above is really the case, mind you, but that's my impression.
-Gecko