Highlander: CSI (that one requires no explanation)
An Immortal Vegas showgirl who solves mysteries on the side.
An Immortal surgical resident who solves mysteries on the side (you're sensing a theme here, right?) and has a lot of sex.
An Immortal mystery authour who solves murders -- a Highlander meets Murder She Wrote sorta thing.
An Immortal version of the O.C.. Instead of catfights, they duel to the death, declaring after victory, "Now, there's one less!" (solving mysteries optional)
A cranky Immortal diagnostician solves mystery illnesses, surrounded by eager young residents.
Feel free to add your own.