The writers aren't afraid to expand minor characters' screen time
There's minor and there's minor.
Robb Stark is one of those characters who is enormously important to the plot
of the novels he just doesn't get as much 'screentime' as the other non-Rickon Starks.
The same can be said of many characters who have had notably expanded roles, like Jaime Lannister and Margaery Tyrell. They're major players in the game of thrones, and thus matter in the big picture in ways that say Ser Dontos or Hot Pie did not.
That said? Bronn's worked excellently as a foil for Tyrion, being written into scenes he's not in in the books and also some of those brand new scenes, so I wouldn't be surprised if he's leaned on in that role next season.