Well, an option would be that Imperial just unlocks five more slots in the deck with cards preselected from the regular deck. It would thus add a new benefit to Age five, without having to create a whole new deck of cards.
Who knows maybe then we would get to see some of the interesting but currently unviable cards getting some action. After all, Age 5 is supreme late game already, and the needs of such a gameplay aren’t exactly the same as early or mid game.