I'd like to see a little more personal relationships.
In previous seasons, we saw Frank have a few girlfriends, Erin get involved with a few guys, and Jamie and Eddie flirt with a relationship. But for the last two or three seasons, there's been practically nothing.
It fleshes the story out to have them be three-dimensional characters, involved with people outside of their job and family. Imagine the stories they could do, with Frank bringing a woman into the family, or Jamie and Eddie getting more serious and jeopardizing their partnership, or Erin finding a guy who's not a douche, or even Danny - eventually - finding someone to replace Linda. Maybe even grandpa can get in on the action.
These characters need to get laid more often.