His story will fit into the “grand finale”.
Questioned by Indiewire about the end of Phase 3 Marvel Studios, Kevin Feige, the big boss of the firm, let go that Captain Marvel will be in Avengers 4: ” We really focus on Captain Marvel and the work that Anna and Ryan do. It will be a big piece that will lead us to the epic conclusion of 22 films over ten years (…) The goal is to make Captain Marvel and then integrate the story of the character in the grand finale. Underlining: Avengers 4 will be following the storyline of Captain Marvel.
Captain Marvel will take place in the 90s and the end should explain why Carol Denvers has not reappeared since. Recently, Kevin Feige hinted that the character has been in space for all these years. His return could be closely related to Thanos.
Avengers 4 will be released in our theaters on May 1, 2019. Avengers: Infinity War is due to land a year before, April 25, 2018.