Avengers: Endgame secretly hinted at Black Widow's origin in the comics. The secretive Russian spy played a major role in Avengers: Endgame as the Avenger who refused to give up. Her journey came to an end on the planet Vormir, where the Red Skull revealed that someone would need to be sacrificed in order to acquire the Soul Stone.
The Red Skull impressed Black Widow with his knowledge, surprising her when he correctly identified her father as "Ivan." At first glance, this appears to be a reference to the comic book character Ivan Petrovich Bezukhov. He was a Russian soldier who became Natasha's guardian when he rescued her as an infant, and he allowed Stalin to place her in the Red Room. Both Natasha and Ivan were injected with the Infinity Formula, a serum that essentially grants near-immortality, and as a result they aged at a far slower rate over the decades. Disturbingly, Ivan's relationship with Natasha became something far more sinister; he became infatuated with her, lusting after her.
However, this reference may actually be a little deeper. As noted in the comics, Black Widow was not born in 1984; she was a child as far back as 1917, when she was orphaned during the Russian revolution. Her comic book creators - Stan Lee, Don Rico, and Don Heck - deliberately gave her the surname "Romanoff" as a hint of her true origin; her name is a Westernization of "Romanov," the surname of the Russian royal family who were killed during the revolution. They were clearly tapping into legends of a surviving Russian princess, Anastasia, which inspired a flurry of comics, novels, and even live-action and animated movies alike.
Viewed from this perspective, Avengers: Endgame's reference to "Ivan Romanoff" seems to be a nod to a legendary member of the Russian royal family who passed away in 1640. Of course, in the MCU Natasha hasn't been injected with the Infinity Serum; in fact, in Captain America: The Winter Soldier, Arnim Zola confirmed she was born in 1984. Presumably her father was named after his famous and celebrated ancestor.
The upcoming Black Widow movie is expected to detail Natasha's MCU origin story, but it's unlikely to include any mention of the Russian revolution of 1917. Those events had been relegated to legend in 1964, when Black Widow was created in the comics, but they've generally been forgotten now, especially after the fall of the Soviet Union. They'd feel odd and anachronous, meaning this subtle reference is probably the best fans are going to get.
