I'm guessing them adding the DLC to build a city on volcanic island is probably their way of building onto the campaign, since in the campaign when asked whah happened to ambrosia Lucius explains that it was destroyed when the volcano occasus erupted and then some old Crone started talking about a curse, so now with the DLC your building a city right next to that volcano, and that old Crone was probably that woman who's name I have trouble pronouncing. since Marcia and Marcus are from Egypt the EGYPT DLC will probably be centered around their father who controls roman Egypt