According to The Hollywood Reporter, Sebastian Stan's Winter Soldier will be appearing in Avengers: Infinity War, which hits theaters on May 4, 2018. With Bucky volunteering to go into cryostasis at the end of Civil War, many fans thought Bucky would exit his cold-sleep in Black Panther's solo feature, which is currently slated to arrive in cinemas on February 16, 2018.
This means one of two things will occur - Bucky wakes up in Black Panther and then goes on to appear in Infinity War (which means this news is a bit of a spoiler) or Bucky stays asleep in Black Panther and doesn't awaken until Infinity War.
Here's where things get somewhat interesting. THR's report on Sebastian Stan's new project, We Have Always Lived in the Castle, mentioned that filming begins this month in Dublin. We previously learned that Black Panther doesn't start filming until early-2017 while Infinity War is scheduled to start shooting this November in Atlanta. The average, non-blockbuster film typically shoots for 10-12 weeks, meaning Sebastian should wrap We Have Always Lived in the Castle in October/November, leaving him plenty of time to appear in both Black Panther and Infinity War.
Stan's Winter Soldier screenwriters (Christopher Markus and Stephen McFeely) and directors (Joe and Anthony Russo) are all returning for new Avengers film, which will finally see Thanos begin to take direct action.