The Golden Age Black Widow Theory's a fun idea, but doubtful at this point. Given that the SHIELD
writing team has been playing fair with the "ItsAllConnected" theme up to now, to bring in a new character in as the leader of the Big Bad who's had no history with the series at all yet?
Not seeing that happening.
The character might be introduced down the line as part of another arc, but she'd be more likely as an adversary in a Doctor Strange