At least two more Marvel characters will join Luke Cage in season 2: Tilda Johnson and John McIver. If you're a fan of the comics, you may know them better by their super monikers: Nightshade (now Nighthawk) and Bushmaster, respectively. Mustafa Shakir (Moses from Quarry) will play McIver and the role of Johnson will be filled by Gabrielle Dennis (Pippy from Rosewood). Marvel broke the news on its official site.
Marvel Comics background
A quick comics lore recap: McIver is a villain through and through. Hailing from the Caribbean island of Saint Croix, he went from small-time criminal to crime boss with a little help from a modified super-soldier serum (the same stuff that gave Captain America his powers). He's way into Misty Knight so he'll probably end up butting heads with Iron Fist at some point, too.
Johnson's relationship with superhero-dom is a little more complicated. She was born into a poor Harlem family and initially turned her astounding intellect toward a lucrative life of crime, even partnering with the Yellow Claw and adopting the name "Deadly Nightshade". After bottoming out with the whole villain thing, Nighthawk saved her from a mugging and she put her talents toward his heroic cause. From then on she became a vigilante and hero, eventually donning Nighthawk's mantle herself.
Marvel Cinematic Universe incarnations
According to the Marvel announcement, the Netflix Luke Cage incarnation of John McIver is "a natural leader, brimming with charisma, whose mission is focused on Harlem and vengeance." Charisma and vengeance? Sounds like he's probably still a bad guy, then.
Johnson is "a brilliant, holistic doctor with a complicated history in Harlem where, as much as she tries to stay far from trouble, it seems to always find her." So she'll probably have outgrown her villain days by the time we meet her on television.