A is Afro-American in 2018 and decides to participate in a scientist's experiment on time travel, only to be projected back into 1400s England...where they meet B, a very attractive and very single English citizen.

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Alternatively...
A meets their partner in the modern times (B), an immortal who's never revealed their power to anyone.

Can be reversed & ethnicities can be more different! This prompt is originally very specific and could sound kind of racist, but it's only because in this idea I originally focused on the language barrier that exists between them (slangs vs traditional, old English).

God that sounded like a textbook trying to sound politically correct haha sorry