Avengers: Infinity War - Every Character Confirmed So Far

Confirmed By: The supposed "official casting call" confirmed she's involved, which should come as no surprise to anyone paying attention to the plot resolution of Civil War. Widow is on the run at that point, and presumably driven underground to meet the other Avengers, so it would be odd for her not to reappear.

Plot Predictions: The start of the film will likely see her immediately following on from her being forced underground on the run from the government's pro-registration forces. Whether than means she'll gravitate towards Nick Fury or Cap and his other Avengers remains to be seen, but you'd have to think they'd have some trouble trusting her.

The ins and outs of that might be uncertain, but she'll obviously then get back together with the Avengers when Thanos' attack fixes their bonds as a team.