Marvel is definitely in the business of collecting talented actors for roles in their franchise. Aftrr Benedict Cumberbatch was announced for the titular role in the upcoming Doctor Strange, it only seems natural that his costar in BBC's Sherlock would be quick to follow. But Marvel excecs are keeping a tight lid on the details of Civil War, which is currently in production. So while we know that Freeman has been cast, its not clear yet who he'll be.
According to The Latino Review, Freeman might be playing one of the instigators of the Superhuman Registration Act, one of the catalysts for the Avengers' internal schism.
"The hot rumor that we've been hearing (this is unconfirmed because NO ONE wants to confirm this at the moment) is that Martin Freeman is playing a character named Edward Chase, the Prime Minister of England in the MCU. Whatever mission goes south for Captain America's Avengers at the beginning of the movie will involve a number of British soldiers, so Chase (Freeman) will be responsible for upping the pressure on the government side, even though it is an international government body. Chase will be arguing for a better world-wide plan to keep The Avengers in check."
It's almost impossible to confirm rumors like these thougn. Marvel has been very aggressive about stopping leaks, going so far as to subpoena Google to find out more information about a user who leaked the trailer forAge of Ultron a week ahead of schedule. According to Ben Beaumont-Thomas with The Guardian:
"Google must now bring “all identifying information for the user ‘John Gazelle’” to a San Francisco court on 18 November, including “the IP address from which the file named ‘ap-tlr-l_int_360p.mp4’ and containing content known as Avengers: Age of Ultron was uploaded to the Google Drive Site” – the IP address will help locate where the file was uploaded from."
It seems reasonable to assume that Freeman will be playing an antagonist or villain. Marvel has historically hired big name actors for one-off roles (Lee Pace in Guardians and Robet Redford in Winter Soldier), capitalizing on the popularity of the actors without locking them into a lock term contract. And lately Freeman has been a familiar face in the nerd community; his portrayal of Bilbo in the Hobbit trilogy just increased his noteriety.