When Black Widow's solo movie lands in Phase 4 (or whatever the post-Infinity War format is going to be known as) it's likely that Natasha Romanoff won't be the only familiar character who turns up. As the established logic is that we're looking at a prequel for the stand-alone that follows her training as a Widow agent and leads to her defecting to SHIELD.
It's a fairly good bet that we'll see both Nick Fury and Bucky Barnes in there as their histories overlap (and Agent Carter and Howard Stark MIGHT be possible if they haven't retired by then), and Agent Coulson is a smart bet too. And apparently, Kevin Feige has heard lots of rumours relating to the list of potential characters (via ComicBook.com):
“There are a lot of potential characters that are among that list.”
But one character who you probably SHOULDN'T think is going to appear is Daredevil, who has been a long-time love interest to Natasha in the comics. When asked about him, Feige said simply “That’s one I haven’t heard."
He's heard. He doesn't mean he hasn't heard. He means it's not happening, but in the most coy way possible. And why would it happen? Marvel have repeatedly said that a crossover with the Netflix shows isn't possible, and it's not like Daredevil could be in Black Widow's past anyway when he only decided to be the hero Hell's Kitchen deserved in the present day.
Still, it would have been a nice little link all the same.