Executive Producer/Marvel’s Head of Television, Jeph Loeb spoke on Simmons return as Mack:
Mack’s story from Season 2 left him at a great place for us to continue, opening up new possibilities for the character as he takes charge of overseeing.
So, a bit like Warehouse 13, but awesomer.
Marvel’s Agents of S.H.I.E.L.D. third season returns Fall 2015 on ABC