Unfortunately, the 2 additional classes for the expansions only apply to their respective campaigns. Meaning, you can't use Assassin/Ritualist on a nightfall or prophecies campaign. Not in the beginning anyways. There's a way to port over to the prophecies campaign from Cantha and quite possibly, there would be an option to travel to Elona now as well.
Also, there has been an estimated 2 expansions coming out per year. This year being Factions and now, Nightfall. With each expansion comes a brand new campaign, and 2 new classes to choose from, as well as some new equipment and skills probably.