Mackie himself has personally confirmed he is indeed playing Sam Wilson, a.k.a The Falcon, in the Cap sequel.
"I’m playing the Falcon. He’s this guy in Harlem who moved to California and became a drug dealer," Mackie said to News Day. "His plane crashed, and he was genetically altered, and he can fly, has telepathic powers.
"He’s the first African-American superhero. It makes me feel all the work I’ve done has been paying off. I have a son, nephews and nieces, and I love the idea that they can dress up as the Falcon on Halloween. They now have someone they can idolize. That’s a huge honor for me."
After The Avengers was released in theaters this past May, writer/director Joss Whedon made it a point to say that Marvel's cinematic universe needed more gender balance and diversity. Bringing Mackie into Cap's next movie certainly means Whedon was serious about what he said. Plus, The Falcon is a S.H.I.E.L.D. agent so fans could see more of him outside of Captain America: The Winter Soldier and helping S.H.I.E.L.D. whichever way he can.