I had this idea a while ago, but it was just sitting unused in the back of my mind.
Supreme + imperium = Superium or Supreme + dominion = Superion
Not just an empire. It's comprised of several empires. The Imperator is the supreme ruler of the Superium, with "lesser" emperors governing each domain.
I didn't know where else to take this draft.....until Astrographer posted about Galactic Empires. And that is just the first link in a spectacular post loaded with useful information.
Mind blowing stuff, all of it!
It got me to thinking 'if the reach of the intergalactic government is too broad to be more than marginally effective, how do you get around that and rein it in?' Easy - reign it in. The Imperator chooses not successors, but co-rulers. The Roman Caesars didn't necessarily pass their title onto family members, as would a monarch or feudal lord, they would select who was to reign when they were gone. That is of course an over-simplification. Politicians are known for scheming more than doing any actual good for the empire.
My idea is that the Imperator's "chosen ones" are given whole sectors of the galaxy to run as they see fit. They are more powerful that the emperors, but just below the power of the one who chose them. They must still answer to him. Therefore, they must do what they can to maintain their area, or else be stripped of their title and holdings. But there's nothing saying they wouldn't try to forge an inheritance that names them the new Imperator once the current is.... removed. The lingering question is "how many?" That depends on how much I, or you, choose to devote to the politics of it all. 2 or 3 under the Imperator would improve efficacy, while more than say 5 or 6 wouldn't give them much more power than the regional emperors. You decide how many based on how large the Superium is.