Birkhoff finds Nikita's birth certificate.
I'm not kidding.
my guess is there's two possible outcomes:
a) we learn that "Nikita" isn't her real name.
b) Nikita is really an Udinov, and has a better claim to the throne of that Russian supercompany than even Alex does.
okay, technically there's three possibilities. the third is
c) Birkhoff doesn't tell anyone until the season finale, when he reveals that Nikita's real name is Nikki.