It's announced that the Marvel Comics character Barbara Morse, aka Mockingbird, will appear this coming season.
It's also been announced that the Marvel character Kraken will appear, and Lucy Lawless, who was previously announced as part of Season 2, will be playing S.H.I.E.L.D. veteran Isabel Hartly. It was stated Nick Blood will play Lance Hunter.
Update: The gag reel has officially been released.