While you are at it, I'd modestly recommend you to add "temi (pri)". When I finished the course and started reading real world texts, I was surprised by the frequency of it; it seemed to me that no one writes more than 3 paragraphs without using that. I asked myself "how could they forget to add to the course such a frequent and idiomatic expression?". It is certainly used more than "ebena", maybe unless you are a farmer, geologist or real estate agent :p
Well, it could be useful in historical context, or for fictional works and fairy tales (like The Emperor's New Clothes)
And, since imperiestro literally means "empire leader" it's a little broader than the English word Emperor. In Esperanto it could also be used to refer to kings and queens in the days of the British Empire, or the Spanish Empire, or the Dutch Empire, etc. if you want to emphasize their positions as leaders of an empire over their position as head of a monarchy.