Perhaps mysteriously and suddenly go missing for 48 hours then turning up with no memory of what happened to him.
I don't want him to have a memory loss. that's too soapy in my opinion. I'd rather he just have a case that gets to him like cases have gotten to Nick or maybe the vic turning out to be someone he knew back in high school (and yeah I know he went to school in a different state, I'm just saying for example).