In the comics, Everett Ross is a U.S. State Department employee who acts as a liaison to foreign diplomats, specializing in the country of Wakanda. In the film, Ross is a government official who is involved with the manhunt for Bucky Barnes and by association, the Sokovia Accords. Speaking on The Graham Norton Show, actor Martin Freeman says his small role in Civil War is just a precursor to bigger things later on in the MCU. "I think my character will develop a bit more as I enter the Marvel world a bit more which seems to be happening."
Examining Freeman's remark reveals that he's learned a bit more about where Ross appears next. Given his comic book counterparts association with Wakanda and T'Challa in particular, it's likely that Freeman shows up in the Black Panther movie, set for release on February16, 2018. However, [without going into spoilers] the ending of Civil War provides a clue that we're likely to see Ross in one of the four films that arrive before the Prince of Wakanda makes his solo feature debut.
As far as how small his role in Civil War actually is, Freeman joked with fellow Graham Norton guest Paul Rudd, "Apparently [Paul and I] are in the same film but our paths didn’t cross, in fact my path didn’t cross with an awful lot of people in that film. I watched it for the first time today and I didn’t know anyone in it!”