An old mentor of Rozzoli's would be an interesting character to include. This mentor could have retired but still has a lot of influence on Rozzoli. He could show up to give advice on a particularly difficult case, and his presence could stir up old memories and emotions for Rozzoli. His relationship with Isles could also be an interesting aspect to explore, as he might initially be skeptical of her but then come to respect her skills.
A new detective in their department could be a great character. This detective could be young and eager, but also a bit of a rival to Rozzoli. He might have different methods of solving cases and could create some tension in the team. At the same time, he could also develop a friendship with Isles, which might make Rozzoli a bit jealous.