Roman-History Themed Set (Ascalus)

Yeah, that would probably be a better wording...

the thing is that Theros isn't one-to-one Greece, it's the loose points of Greece everyone knows through a fantasy funhouse mirror with the bits in the middle filled with original stuff. you don't need to adhere to specific historial/mythological minutiae in a translation into a plane (and if Kamigawa's any indication, sometimes doing that can be a detriment to your project). it's not iron, it's putty, is what I'm saying

With the Emperor mechanic, what exactly is it doing other than acting as a toggle state for other cards? Because if the only criteria is that it be a toggle state, only one person can have it at a time, and it doesn't stay on them forever, well, the Monarch already meets those criteria. A king, an emperor, a czar...call it what you will, they're all just titles of monarchs. So you could easily have a mechanic; Election (The player with the most cards in their graveyard becomes the Monarch.)

With a variation on permanents of either "During each upkeep...", or on permanents write out "Each [upkeep, end step, time a card goes to the graveyard, etc...] hold an election."

Or, is there something you want the Emperor state to do that the Monarch state doesn't? Or something you don't want it to do? If an existing mechanic already does something you're trying to do, there's no sense in reinventing the wheel. Using something familiar anchors your set and makes it easier for people to get into it. I'm not saying don't use your own mechanic, but have a clear grasp on what it does differently and why it's different. Be able to convey that clearly to others in your design, so they immediately grasp what sets it apart.