Not about Age of Ultron, but still about Black Widow and Iron Man. Comic book versions, not film.
Probably not any big surprise, but I'm expecting that the big secret Natasha didn't want revealed by Weeping Lion is that she killed Tony's parents. I don't know if that means Howard and Maria, or his biological parents - who Tony may not have discovered the identity of yet, I don't know, I'm not reading International Iron Man - but one set of them. Which would explain Black Widow #5 ending with pissed off Iron Man hunting for Natasha.
Although, there is another possibility. Highly unlikely, and probably very ill-advised, but what the hell: Natasha is one of Tony's biological parents. She's much older than she appears - Waid and Samnee have already referenced this in issue 4 - and as far as I know, the comics have not incorporated Whedon's decision to have her surgically made incapable of conceiving a child. It's feasible, though I wouldn't advocate for it, since I don't think it's really a development either character needs. But I didn't see the point in revealing Howard and Maria aren't Tony's biological parents, either.
It was just a notion that came to mind a couple of weeks ago.