I am a Star Trek fan (yes, a Trekkie), and I think Tim Russ who played the Vulcan Tuvok would be great in this show as a recurring or one off character as a brother or relative of Holt, who does a modified character like Tuvok.

Just, Russ would be excellent as a brother, or a brother who is in the NYPD, who is more unflappable than Holt. Trekkies will understand.