I think "concubine" is fine. It reflects history and reality, virtually all powerful men have one or more, and it is also a simple acknowledgement of the truth that the hypergamous woman would rather have 1/10th of the powerful man than 100% of a man closer to her own level.
But since feminists fear and hate this truth (among many others) I expect the forced societal usage of some faux "empowerment" label, instead. :)