This is the 2288th most frequent Russian word.
кем
The Russian word "кем" is the instrumental case of "кто" and means "by whom" or "as whom."
In this question, 'кем' is used in the instrumental case to ask about someone's profession or role.
Кем ты работаешь?
Who do you work as?
In this context, 'кем угодно' is plural and instrumental, suggesting flexibility in the role to take.
Ей предложили стать кем угодно, кроме директора.
She was offered to become anyone except for the director.
Here, 'кем-то' is used as an instrumental case form of 'кто-то' to indicate an unspecified important role.