It's revealed that Sebastian Stan will be returning as Winter Soldier for Avengers: Infinity War.
In THR's report on getting cast in We Have Always Lived in the Castle, it's mentioned Sebastian Stan will shoot Avengers: Infinity War early next year.
Last we saw of the Winter Soldier, he was back in stasis under the watchful eye of Black Panther as shown at the end of Captain America: Civil War. With the info about Sebastian Stan appearing in Avengers: Infinity War coming out, we can speculate that possibly Winter Solider may be freed in the upcoming Black Panther movie, or it's possible Winter Soldier could simply be freed to help out the heroes in Avengers: Infinity War.
Sebastian Stan hasn't yet been confirmed for Black Panther, which hits February 16, 2018, followed by Avengers: Infinity War's May 4, 2018 release.