One of the things Agents of Shield seriously did right with its second season was adding a bunch of new team members to grow the dynamic. One of the biggest was Henry Simmons as Mack.
Mack had a big part in the later part of the season, and got promoted (in-show) after helping stop the Inhuman attack. Now, it appears the promotion will become literal, as he’ll be a regular in season 03. No doubt he’ll have just a big a role when it returns this fall.