Aside from the fact that he could be an anscillary character made up especially for this film, there are a couple ideas floating around the BSR! staff here.
He could possibly be playing Sam Wilson (aka The Falcon) – long time friend and ally to Captain America. However, The Falcon came later in Steve Rogers life after he arrived in the present time and wasn’t around in the 1940′s. But, no one knows what the continuity of this film is going to be, they could introduce him as a character during WWII with Cap to help drive the story. Personally, I think this one is unlikely.
Perhaps he is playing Isaiah Bradley (or a similar character) from the Captain America “Truth: Red, White, and Black” series. In the early stages of the Super Soldier Serum testing the military used a small group of African-American soldiers as test subjects prior to injecting Steve Rogers. In this canon Isaiah Bradley becomes the “original” Captain America of sorts. So perhaps the film brings this back story in and Luke is the main focus on that.
Finally, another popular idea around here – and Mr. Vice’s movie-sense is tingling on this one - is that he could be a leading member of the Howling Commandos, Gabe Jones. An elite military unit formed during WWII.
There is no information anywhere about Derek Luke being in this film, so it was quite a surprise to see his name pop up on the poster. Personally, I’d like to see the film incorporate the “Truth” series a little bit and Luke would be a great fit as at least a similar character to Isaiah Bradley. In any case, we will have to hang tight to see if any information surfaces about what possible role he will be playing, or just wait for the movie!