Black Widow and Hawkeye made their Disney Parks debut over the weekend, appearing at Disneyland for the Avengers Super Heroes Half Marathon. According to the Disney Parks Blog, the were joined at the event by Thor, Captain America and a contingent of S.H.I.E.L.D. agents.
If you missed them, you may be out of luck, as Disney's Shawn Slater there are no plans for Black Widow and Hawkeye to visit Disneyland on a regular basis. However, Captain America, Thor and Iron Man maintain a presence at Innovations in Tomorrowland (interestingly, ROBOT 6's J.K. Parkin noted that Iron Man was nowhere to be seen at the Half Marathon).
Captain America was a bit of a trailblazer on that front, as his debut at the D23 Expo in August 2013 marked the first time a Marvel costumed character had appeared at a Disney event.