If rumors are to be believed, Holy Fury is the last CK2 dlc, and ck3 is a possibility in the next few years, possibly even their next major project, so its not impossible they push the date for that all the way back to the collapse of the Western Roman Empire in the 5th century. I don't know if its realistic to expect DLCs for imperator Rome to extend 600 years ahead, but we'll see.
The 5th century doesn't fit into a CK setting. It was a completelly different era with different mechanics.
Also you do the same. You expect a 600 years (1066 > 5th century) expansion for CK.