According to the official Star Wars website, we're going to be following a new deadly gunfighter whose story is not unlike Jango and Boba Fett's. The initial announcement said we'll "follow the travails of a lone gunfighter in the outer reaches of the galaxy far from the authority of the New Republic."
The series will be set in hitherto unexplored parts of the Star Wars universe, taking place after the fall of the Empire (Episode VI: Return of the Jedi) but before the rise of the first order (Episode VII: The Force Awakens). Could some of the characters from the new trilogy possibly make appearances at some point? Fingers crossed.
As for who's starring in the series, Pedro Pascal (perhaps best known for playing Prince Oberyn in Game of Thrones) will portray the "lone Mandalorian gunfighter" teased in the premise. Also in the cast are Gina Carano, Giancarlo Esposito, Emily Swallow, Carl Weathers, Omid Abtahi, Werner Herzog, and Nick Nolte. Whew, that's a lot of star power! Keep your eyes peeled for the group when the series premieres in Fall 2019.